Coldwater Aquariums

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Oranda Goldfish

This is a guide designed to help those wishing to go cooler than the well known tropical tanks. This guide details fish both classed as "coldwater", tolerating temperatures down to around 10°C (50°F) and "sub-tropical", tolerating temperatures from around 15.6°C (60°F) to lower tropical temperatures around 21.1-23.9°C (70-75°F) . But, as with all tanks, there's a lot more to keeping these fish than just temperature!

If you are considering going "coldwater" as you have decided this will be easier than keeping a tropical heated tank, unfortunately you've been misguided! To class a tank truly as coldwater you would need to have a chiller installed. A chiller costs substantially more to buy and run than a heater! If you are going to get your first tank, I strongly suggest buying a heater and either setting it low for sub-tropical fish, or going the whole hog and going tropical. For fish to thrive they need stability, this can't be maintained in an "unheated" room temperature tank, unless you have a room that is temperature controlled of course.

Be aware that a lot of coldwater animals do have legality issues in temperate countries such as the UK and Australia, simply because, although they're not native, they would survive in the wild waterways, destroying populations of native fish.

If you still want to stay chilled, have a look at some of the options below. There's a lot more out there than just Goldfish!

Freshwater Fish

These are just some of the many options of freshwater fish you can find, with more coming into the hobby all the time. These fish are listed in order based on minimum tank size needed, smallest to largest.

  • Adult Size: 2.5-3.8cm (1-1.5")
  • Temperature Needed: 15.6-22.8°C (60-73°F)
  • Tank Size Needed: at least 38-57 Litres (10-15 US G.)

This is a hardy little fish which is fairly common in aquatic stores these days. It is a peaceful and active shoaling fish which should therefore be kept in groups of at least 5-6. Albino and long-fin varieties are also available. They thrive in very well oxygenated water with a strong current. As their name suggests, they originate from fast-flowing mountain streams. To get the best out of these fish it's best to replicate their natural environment.

  • Adult Size: 5.1-7.6cm (2-3")
  • Temperature Needed: 17.8-25°C (64-77°F)
  • Tank Size Needed: at least 57 Litres (15 US G.)

Unlike most Corydora Catfish which are tropical fish, the little Peppered Cory prefers much cooler temperatures, which makes it ideal for a sub-tropical community set up. Like most Corydoras, the Peppered Cory loves company of its own kind and should be kept in groups of at least 4-5. Corys require a soft substrate in order to keep their barbels healthy, sand or very fine gravel is ideal and will mean you get to see its natural digging behaviour. These are exceptionally peaceful fish that will not bother tank mates, indeed they'll troll around the bottom of the tank oblivious to any other occupants. They will struggle in very strong currents, best kept in lightly planted tanks, or with plants that don't mind their roots being disturbed, with equally peaceful sub-tropical fish.

  • Adult Size: 5.1-10.2cm (2-4")
  • Temperature Needed: 17.8-25°C (64-77°F)
  • Tank Size Needed: at least 57 Litres (15 US G.)

These are curious little fish, also called UFO Plecos and Hong Kong Plecos, which means many mistake them as Pleco Catfish. They are not, they are part of the River Loach family. As their name suggests, they originate from fast-flowing mountain rivers, very similar environment to White Cloud Mountain Minnows which makes them ideally co-habitants. These fish thrive in mature tanks with large rounded river rocks to cling to and a strong current with a lot of oxygen in the water. Plants aren't really required as they wouldn't come across much in the wild. The tank must be mature though with some algae growth on which to graze. It also seems to enjoy company of its own kind.

  • Adult Size: 2.5-5.1cm (1-2")
  • Temperature Needed: 11.7-25.6°C (53-78°F)
  • Tank Size Needed: at least 57 Litres (15 US G.)

This is an active little livebearer that should be kept in the livebearer ratio of a least 1 male per 2-3 females. This fish is a surprisingly aggressive little fin nipper, it should only be housed with fast swimming and robust fish, fish like Zebra Danios, White Cloud Mountain Minnows or Rosy Reds should be able to tough it out with them. As this fish is a livebearer females will produce fry monthly, but are likely to eat their own fry if the fry aren't given enough cover. They are fairly hardy and will tolerate both freshwater and brackish. Food wise, they are unfussy accepting most small meaty food like brine shrimp and daphnia, they may also nibble at plants too. They are the most colourful of fish, with females looking very similar to Guppy females. Males have shimmering blue iridescence on their flanks and are substantially smaller than females.

  • Adult Size: 3.8-5.1cm (1.5-2")
  • Temperature Needed: 20-25°C (68-77°F)
  • Tank Size Needed: at least 57 Litres (15 US G.)

This is a very active small livebearer, something a bit different from the common Guppy or Platy, but just as prolific! As with all livebearers, this fish should be kept in the ratio of 1 male per 2-3 females to avoid the females being pestered and bullied by the over-amorous males! They are surprisingly attractive little fish too, looking fairly plain at first, both males and females have metallic iridiscence and faint vertical banding on their flanks, as well as shimmering blue eyes. The males get the "Black Chin Livebearer" name from the large amount of black from the base of their mouth right down to the tip of their elongated gonopodium and along the base of the body to the caudal fin. Both males and females also have a black spot at the base of the dorsal fin. They may also have black spots around the top of the head too. They are very active and are therefore best in tanks over 57 Litres (15 US G.) despite their size. They also don't generally eat their own fry, but dense hiding places in java moss will help protect the babies when they're born. These are peaceful and active fish that work with similar sized fish and inverts, they should not be kept with larger fish which may eat both adults or fry.

  • Adult Size: 7.6-12.7cm (3-5")
  • Temperature Needed: 15.6-23.9°C (60-75°F)
  • Tank Size Needed: at least 57 Litres (15 US G.)

Also known as the "Paradise Fish", this fish does not live up to its angelic name! These are very territorial sub-tropical Gouramis that are best kept in spacious species tanks. They work well in a male/female pair, provided they've bonded, and should not be mixed with any other anabantoid or equally territorial species of fish, if any. They may work, in a big enough tank, with fast swimming fish such as Zebra Danios, provided there's enough swimming space for both. That said, male Paradise Gouramis can be stunning fish with several colour morphs now available and with large flamboyant fins. A definite colour splash in a planted tank! They do not tolerate strong currents however, a heavily planted tank with plenty of places to explore and hide with a soft current is ideal.

  • Adult Size: 5.1-7.6cm (2-3")
  • Temperature Needed: 18.3-23.9°C (65-75°F)
  • Tank Size Needed: at least 57 Litres (15 US G.)

When most people think "Tetra" they think of strictly high-end tropical Tetras, such as Cardinal Tetras, but there are exceptions and the Bloodfin is one of them. Like other Tetras, the Bloodfin is a shoaling Tetra and must be kept in groups of 6 or more. They are known to fin nip so it is best to keep these with short-finned fast swimming fish and to be kept in a tank with plenty of hiding places and planting for them, and tank mates, to hide. They are otherwise fairly peaceful and active, and with their striking red fins, make for attractive fish in a sub-tropical tank.

  • Adult Size: 5.1-7.6cm (2-3")
  • Temperature Needed: 17.8-25°C (64-77°F)
  • Tank Size Needed: at least 57 Litres (15 US G.)

The Buenos Aires Tetra has very similar care needs to the Bloodfin above. The Buenos Aires must be kept in shoals of 6 plus and should not be kept with long-finned fish as they may well fin nip. Unlike the Bloodfin, they only have red in the caudal and anal fins, with a black marking through the caudal peduncle too. They are also available in an albino morph.

Long-fin Zebra Danio
  • Adult Size: 5.1-7.6cm (2-3")
  • Temperature Needed: 18.3-23.9°C (65-75°F)
  • Tank Size Needed: at least 76 Litres (20 US G.)

The Zebra Danio is an exceptionally well known and easily found fish in aquatic stores. It is a very active and relatively hardy shoaling fish. They do often get marketed for small tanks which, sadly, are not suitable. Due to this Danio's active nature and need to shoal, they need at least 61cm (24") long and to be kept in a group of at least 6. Any less and in any smaller a tank and they are liable to turn on one another and any tank mates. In a spacious tank with an equal mix of planting and open swimming space these are excellent community fish, as long as fellow tank mates are not bothered by this fish's very active and bold nature. There are a few morphs available of this fish, both albino and long-fin have been seen. It's cousin, the Leopard Danio, which is said to simply be a colour morph of the Zebra, would also be suitable.

  • Adult Size: 5.1-7.6cm (2-3")
  • Temperature Needed: 17.8-23.9°C (64-75°F)
  • Tank Size Needed: at least 76 Litres (20 US G.)

Bitterlings seem to be relatively unknown in the pet trade yet are becoming more readily available, especially Asian Bitterlings such as the Rosy Bitterling. This is a small and slightly shy fish that does best in shoals of their own kind, at least 6. They can look fairly pale and washed out in stores, but once they are established in their tank they develop a lot more colour and can look a lot like Melanotaenia praecox from a distance! They prefer a cool tank with a sand substrate and lots of rocks/bogwood for cover. They prefer alkaline conditions to acidic. They are peaceful and should work well with other sub-tropical community fish such as Rhinogobius Gobies.

  • Adult Size: 7.6-15.2cm (3-6")
  • Temperature Needed: 17.8-25°C (64-77°F)
  • Tank Size Needed: at least 76 Litres (20 US G.)

The Rosy Barb is one of the more peaceful Barbs available, as Barbs are well known for being fin nippers, although it is still best to avoid keeping them with long-finned fish, just in case. It is happiest in groups of at least 6, with a few more females than males. They originate from Asia and are fairly unfussy about the tank environment, tolerating a range of water chemistries. They prefer a spacious tank with a good amount of open swimming space and some places to dash and hide if necessary. They prefer a soft substrate of fine gravel or sand. They are even fairly simple to breed in captivity as long as there's nothing around that could eat their eggs. They generally get between 7.6-10.2cm (3-4"), but 15.2cm (6") wild fish have been known! They are fairly robust and will tolerate the active nature of similar sized fish such as Zebra Danios.

  • Adult Size: 7.6-10.2cm (3-4")
  • Temperature Needed: 17.8-23.9°C (64-75°F)
  • Tank Size Needed: at least 76 Litres (20 US G.)

This fish is commonly sold as a "feeder fish", which it isn't really suitable for. To the untrained eye they can even be mistaken for juvenile Goldfish! They are happiest in groups of at least 4-6 of their own kind and should be peaceful towards other sub-tropical species of fish. There are several colour morphs of this fish, the wild variant being olive with a dark lateral line, the other morphs includes orange and a pinkish variant which gives them the common name "Rosy Red". They do best in a spacious tank with plenty of rocky hiding places.

  • Adult Size: 5.1-7.6cm (2-3")
  • Temperature Needed: 13.9-23.9°C (57-75°F)
  • Tank Size Needed: at least 95 Litres (25 US G.)

Bottom-dwelling freshwater Gobies are becoming increasingly more available in the pet trade. The Candidus Goby is peaceful towards tank mates, except if they're other Gobies. It's best to keep one to a tank, or a male/female pair may work if the tank is large enough. Their tank should be a minimum of 61cm (24") in length with a gravel/sand mix substrate and hiding places in rounded river rocks and bogwood. They are a current and high oxygen loving species so should work well with fish that like the similar conditions such as White Cloud Mountain Minnows. They are bottom dwelling and they may dig in the substrate, so all décor should be secure. Water quality should be excellent and they are best only kept in well matured tanks.

  • Adult Size: 5.1-10.2cm (2-4")
  • Temperature Needed: 15-25°C (59-77°F)
  • Tank Size Needed: at least 114 Litres (30 US G.)

The Red Shiner is a very active temperate fish that must be kept in shoals of at least 5 of their own kind. They are active swimming fish so require a very long wide tank, a minimum of 88.9cm (35") in length, to allow for this movement. They should never be housed with slower moving fish like Goldfish as the Red Shiners are known to fin nip. They can work with other faster swimming fish like Zebra Danios. Their tank should be well oxygenated with good surface movement. These are an attractive alternative to common coldwater fish, the males are iridescent blue/green and red in colour, the females have less to no red and just iridescent blue/green flanks with hints of red in the fins. The vibrant nature of their colours will show best under excellent aquarium lighting and they can look dull in shops.

  • Adult Size: 15.2-30.5cm (6-12")
  • Temperature Needed: 10-22.8°C (50-73°F)
  • Tank Size Needed: over 76-208 Litres (20-55 US G.) per fish depending on variety

The most well known coldwater fish, the Goldfish is often touted as a beginner option. Sadly it's not that simple to keep and requires very large housing with excellent filtration, it's an exceptionally messy fish! They are best kept in species tanks, not mixed with other species of fish or invertebrates, and are best either kept singly or in groups over 4-5. In pairs or trios they are likely to bully and fin nip one another, especially when spawning season comes around. It is also strongly recommended to not mix long-bodied Common type Goldfish with shorter-bodied Fancy Goldfish. To keep 5 Fancy Goldfish such as Orandas, you ideally need at least a 227-246 Litres (60-65 US G.) tank with a good filter but without a strong current as they are not strong swimmers. To keep 5 of the larger more active Common-type Goldfish you'd need closer to 454 Litres (120 US G.) or more, these fish are best in ponds though which should be over 1893-2271 Litres (500-600 US G.).

  • Adult Size: 17.8-40.6cm (7-16")
  • Temperature Needed: 0.6-21.7°C (33-71°F)
  • Tank Size Needed: over 208 Litres (55 US G.)

This is a large growing North American fish, a little Cichlid like in appearance and behaviour. It will eat smaller fish and does best in a large species tank with others of its own kind, in the wild they are found in shoals of over 20-30 individuals, or with similar sized North American fish. They are not fussy eaters and will take most meaty foods. They are messy and need a very well filtered aquarium. Note: this fish is illegal in England and Wales.

  • Adult Size: 20.3-25.4cm (8-10")
  • Temperature Needed: 10-23.9°C (50-75°F)
  • Tank Size Needed: at least 208 Litres (55 US G.)

This is an unusual eel-like fish often found lurking around the bottom of Goldfish tanks at pet stores, usually when they're only 7.6-12.7cm (3-5"). They are not ideal tank mates with Goldfish however. Weather Loaches are so named as when the air pressure changes, such as an incoming thunder storm, they will go mad, dashing about and crashing into everything. For this reason they need a secure tank with quick tank mates, they will crash into slower moving tank mates and could cause internal injuries. If they find the wall of the tank they will go up, and out if the tank is open topped! They do best in a spacious tank with a good amount of hiding places and for it to be well oxygenated. They do enjoy a good dig, so soft sandy substrate is beneficial, they will also potentially uproot plants.

  • Adult Size: 25.4-40.6cm (10-16")
  • Temperature Needed: 3.9-21.7°C (39-71°F)
  • Tank Size Needed: at least 284 Litres (75 US G.)

This is a big fish that is a bit Cichlid-like in its appearance and behaviour, much like it's cousin the Lepomis macrochirus. Unlike L.macrochirus, the Pumpkinseed is a bit of a meanie, it will definitely eat fish small enough to be considered a snack and can be aggressive towards other tank mates. It can only be kept with similar sized and VERY robust tank mates, including their own kind although two males will display and fight with one another. They are also quite messy fish and require a spacious and well filtered aquarium. Diet wise they are unfussy, most meaty foods will be accepted. They will also take great pleasure in redecorating their tank, so any décor should be heavy and very secure and any heaters/thermometers must be protected. Note: this fish is illegal in England and Wales.

  • Adult Size: 17.8-40.6cm (7-16")
  • Temperature Needed: 10-26.7°C (50-80°F)
  • Tank Size Needed: at least 341 Litres (90 US G.) plus

This is a large Catfish with an equally large mouth to match. It has a very typical "catfish" look with a broad flat head and a large mouth surrounded by several sets of barbels. They are best kept in large species tanks or with fish too large to be eaten. They are primarily nocturnal and are not fussy eaters. They do like to hide during the day and décor should be large and sturdy so it cannot be knocked over and it will uproot plants. These are remarkably hardy fish surviving in conditions that would kill most. This is not an excuse, however, to keep them in an inadequate set up! They are best kept in dimly lit, very well filtered and oxygenated tanks.

  • Adult Size: 76.2-101.6cm (30-40")
  • Temperature Needed: 17.8-22.8°C (64-73°F)
  • Tank Size Needed: at least 757 Litres (200 US G.) plus

Also called the "High-Fin Banded Shark", this fish is found in stores when it is in its attractive and unusual juvenile stage at around 10.2-15.2cm (4-6"). It has a very tall sail-like dorsal fin with black bands and speckling on a white body. Be wary, this fish has the potential to get VERY large and need exceptional housing, beyond the means of the average fishkeeper. Unfortunately they're rarely seen as adults in captivity, many dying long before they have the opportunity to grow to their full potential. If it does reach adult size it won't be half as attractive as it was a juvenile, with instead a long cylindrical body and a short dorsal fin with fairly dull colouring. It is also known to be a shoaling fish, they are often sold alone which leaves them very stressed and prone to disorders. If you do have room for a very large tank then this fish could be an interesting one to keep and an achievement to get to adulthood. Note: this fish is illegal in England and Wales.

Koi waiting for food
  • Koi (Cyprinus carpio carpio)
  • Adult Size: 61-121.9cm (24-48")
  • Temperature Needed: 1.7-25°C (35-77°F)
  • Tank Size Needed: PONDS ONLY over 3785 Litres (1000 US G.)

Koi are up there with one of the best known fish next to Goldfish, indeed it seems alarmingly common for juvenile Koi Carp be sold alongside Common Goldfish for indoor aquariums. Koi should be considered strictly pond fish, they need very wide, deep and long ponds, a minimum of 91.4cm (36") in depth, and filtered. They also grow exceptionally quickly, easily getting to 30.5cm (12") in a year. On average they get between 61-91.4cm (24-36"), however 121.9cm (48") Koi are not unheard of! Together with a big length they have an equally large girth and a large adult Koi is a LOT of fish. They may only be kept in tanks or large tubs to over-winter as juveniles, these tanks or tubs must be large, ideally over 379 Litres (100 US G.) and very well filtered. Koi are greedy and messy fish, well known for begging for food, be careful not to overfeed though as they can suffer the same digestive problems as overfed Goldfish. Keeping Koi is its own niche market, if you are interested in keeping these fish it would be worth enquiring about town to see if there's a local Koi-keeping club near you, they will be able to help with a pond set up and sourcing good quality fish. In good care a Koi would need to be written into your Will, on average they can last over 50 years, with the oldest on record being over 200 years old!

  • Adult Size: 88.9-127cm (35-50")
  • Temperature Needed: 1.7-20°C (35-68°F)
  • Tank Size Needed: PONDS ONLY over 3785 Litres (1000 US G.)

These are increasingly commonly found in pond stores. They are very unusual prehistoric looking creatures with spines along their bodies and a long snout. They are mostly bottom dwelling and quite shy, most active at dawn and dusk. Due to this, the albino variants are very popular as they're more easily seen in a pond than the darker coloured Sterlets. Sterlets are Sturgeon, the Sterlet is the only fish in the Sturgeon family really suitable for the average home pond, a lot of the others either grow a lot larger and need more of a lake than a pond, while others are endangered or require a more salty set up. Sturgeon roe is often used as caviar which has lent to some of the Sturgeon species' demise. They are peaceful generally (although they may eat small fish and fry) and need careful feeding so as not to die from malnutrition and starvation - if you see them bobbing along the surface of a holding tank at a store, they're already suffering problems with malnutrition! They also require good oxygenation, poor water quality and low oxygen in a pond, especially in the summer, will kill them. These are stunning fish, if you have the space and time for them. Note: this fish is illegal in New South Wales, Australia.

Freshwater Invertebrates

The following invertebrates are suitable for sub-tropical or coldwater tanks. Just like fish, more are becoming available in the pet trade as time goes on. Be aware that all inverts may become snacks to larger fish and there are some legal issues with inverts such as Crayfish.

  • Adult Size: 1.3-2.5cm (0.5-1")
  • Temperature Needed: 20-22.8°C (68-73°F)
  • Tank Size Needed: at least 19 Litres (5 US G.)

These are exceptionally attractive and striking shrimp which are becoming increasingly popular and desirable in the pet trade, with some high graded shrimp selling for extremely high prices! Unfortunately they're not always marketed as needing cool temperatures and so many do not last long in warmer tanks or during heatwaves. They should ideally be kept in cool planted tanks which are several months mature, they are exceptionally sensitive to water chemistry issues. They are really best kept in shrimp tanks and not with fish, due to their tiny size they can easily be eaten by fish, especially their near microscopic shrimplets! In good conditions they will breed readily in aquaria.

  • Adult Size: 1.3-2.5cm (0.5-1")
  • Temperature Needed: 20-25°C (68-77°F)
  • Tank Size Needed: at least 19 Litres (5 US G.)

These shrimp are a much hardier option for shrimp tanks compared to Crystal Reds. They are best kept in a mature planted tank with good algae growth on which to graze. They are sensitive to water chemistry fluctuations. Keep in groups with a good mix of males and females, females are more red and bulky than males. They breed best in cooler temperatures. Provide some dense plants like Java Moss to hide in. They should not be kept with fish that could eat them and are ideally kept in shrimp only tanks. They can be mixed with Caridina shrimp, but should not be kept with other Neocaridina to avoid hybridising.

  • Adult Size: 2.5-6.4cm (1-2.5")
  • Temperature Needed: 20-28.9°C (68-84°F)
  • Tank Size Needed: at least 19 Litres (5 US G.)

The Ghost Shrimp are very similar in size to the Crystal Reds and Cherries, however, they are more scavengers than algae eaters. They will do well on general shrimp pellets and a fairly densely planted tank. Like the other dwarf shrimp, it will be sensitive to water quality. They will also breed well in tank conditions so extras may need to be rehomed. They do appreciate company of their own kind so keeping them in good sized groups is beneficial. They will not hybridise with Caridina dwarf shrimp, but it is best not to mix the species, they have been known to eat the shrimplets of other shrimp species, as well as fish fry.

  • Adult Size: 12.7-30.5cm (5-12")
  • Temperature Needed: 18.3-23.9°C (65-75°F)
  • Tank Size Needed: at least 57-76 Litres (15-20 US G.)

This is a large North American Crayfish that is particularly feisty and aggressive. It will harass tank mates and eat anything it can get its claws on! For that reason it is best to keep one to a tank. Like all inverts it can be sensitive to water chemistry in the confines of a tank so will need a well maintained healthy mature tank with plenty of hiding places in wood and rock. It can be particularly territorial too, so should not be kept with any other Crayfish species. They can also be fairly disruptive in a tank, so décor should be very secure and they will tear up live plants. Substrate should be soft and sturdy enough for them to dig and burrow. Note: this Crayfish is illegal in several countries as it can be an aggressive invader in non-native rivers or lakes. It is illegal in the UK and also a few states in the US, as well as some other countries. There are strict export laws surrounding this species of Cray!

Other Creatures

Leucistic Axolotl
  • Adult Size: 15.2-25.4cm (6-10")
  • Temperature Needed: 15.6-17.8°C (60-64°F)
  • Tank Size Needed: at least 76 Litres (20 US G.) for one

The Axolotl is a Salamander stuck in its larval form. It is a bit of a freak of nature, even in this larval form it breeds and rarely "transforms" into a land-based Salamander. The Axolotl MUST have very cool consistent temperatures, it will suffer in warm water. It is also prone to having it's long external gills nipped by fish and is therefore best kept in species tanks. It will also try and eat anything it can fit in its mouth, including invertebrates, fish and other Axolotls! This could also include their substrate so they are best kept on large river rocks, too large to be eaten, or shallow sand no deeper than 2.5cm (1"). They are very messy and will require a very well filtered, but without strong current, and fully cycled tank. They don't require a deep tank, at least the length of their body in depth should be fine, they will often just perch near the surface, not being overly active animals except at feeding time. They also do not require very heavily oxygenated water, indeed if it's too oxygenated their gills will shrink and wither! Read our article linked above for more details on these creatures.