Considerations in Fishless Cycling

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How it started

Some years ago Dr. Chris Cow, a biochemist and hobbyist, developed a hobby-level technique for establishing that part of the nitrogen cycle important to our tanks without using and abusing live fish. Not the original articles (that site no longer exists), but a good duplicate exists at:

Chris (a.k.a. Nomad) shared his technique with others on a forum that had a number of experienced hobbyists. Several of those served as beta-testers of the technique, and were quite impressed with how well it worked. That core group started popularizing the technique on the other forums they visited, and the rest is history.

Cycling without fish was not novel, several variants had existed for years prior to that time. Some of those variants required knowledge of chemistry and either access to a laboratory or an exceptionally well-equipped home lab, or were completely uncontrolled and frequently smelly. Chris' technique was and is a better fit with hobby materials and required no special lab equipment, only patience and persistence.

The process itself is simple, quite speedy under optimum conditions, and requires little more than materials that a hobbyist should (but might not otherwise) have on hand, plus ammonia. It does require some patience. The speediest completion of the cycle for Chris and most original beta-testers was about two weeks, +/- 2–3 days. The slowest was about a month (~4 ¼ weeks). It also requires the use of liquid-reagent test kits with the ability to read color-based tests (colorblind people may have problems here). We are an impatient society. Few are willing to wait 2–4 weeks before adding fish to their first tank.

This whole concept was intended and aimed at the novice. Folks with other tanks already established have other techniques available to them. If they do opt for fishless cycling of a new tank, they do have ready access to the needed bacteria. Many newcomers could not initially be convinced that the learning experience was worthwhile until after they went through the frustration, massive water partials, fish deaths, and disease outbreaks of conventional fish cycling. Many changed their tune after that harsher experience. The feedback from those early rejections did help boost the newer technique.

There were some unanticipated pitfalls. Those I have run across are the reason for this note. These issues are detailed below:

Clear Ammonia

None of the early testers realized how difficult it could be to find or identify “clear” ammonia. So much ammonia-based product on the market has the sort of additives common to modern merchandising and apparently required for successful product branding.

Detergents or surfactants were expected – much household cleaning ammonia offers a “sudsy” alternative. Perfumes and such came as surprise. We were naïve. Chelators are acceptable, being used for in-bottle stability. I confess that I spent more time and gas orbiting supermarkets, drug stores, and hardware stores than I would have guessed to be required. But the packaging is available, and is most easily found in the store labelled brands or most discounted bottles – these tend to be least marketed and thus the plainest – an advantage for us.

Low-alkalinity or low-KH water

One very embarrassing pitfall was completely unforeseen. We all knew that nitrification uses up KH, alkalinity, a.k.a. carbonate hardness. None of the early testers had low KH water, so we did not see the results of extended fishless cycling on such water. Fishless cycling aims to provide sufficient colony sizes of both needed bacterial forms to sustain a fully stocked tank of mature fish, plus a margin of safety.

Folks trying the technique at our recommendation, who had very soft and somewhat acid water, started reporting what were effectively pH crashes. Very low pH is hard on our nitrification bacteria. We learned, very publicly, that a KH test was required along with ammonia, nitrite, and nitrate tests.


Test the KH of the water that is used for your tank. If the KH is less than 3-4 degrees, or about 50+-70 ppm[1], you may need to supplement the alkalinity or carbonate/bicarbonate with addition of sodium bicarbonate or baking soda[2]. Tropica suggests that one teaspoon (about 6 grams) of sodium bicarbonate per 50 gallons of water will increase KH by about 1 degree and will not increase GH. Do note that such KH modifications should never be done in a tank with fish. The chance of osmotic shock is too great. For occupied tanks, adjust smaller amounts of water outside the tank, such as part of the replacement water for a partial water change, and add that premixed water to the tank slowly when livestock are present. For fishless cycling it is not significant, but try to avoid overdosing.

Chloramines-treated water

There were also a very few cases where chloramines-treated water seemed to interfere with the young colonies at the final large water change before adding fish. This was not fully explored or studied, but seemed avoidable by using only thiosulfate-based dechlorinators if partials were needed during the cycling process and at the final large partial. If this technique is used, wait overnight for any residual ammonia freed by the thiosulfate to be oxidized by the biofilter.

Bacterial inoculation

There still seems to be some confusion on the importance of inoculating with bacteria. Our freshwater nitrification bacteria are quite slow growing for bacteria. This means that colony growth is quite slow with small inoculations. A 'large' inoculation up front will permit full colony development in a couple of weeks. A 'small' inoculation will extend the cycle up to a month or even longer.

But take-home lessons here are two: First, if you add no or only a few of the “right” [3] nitrification bacteria, the fishless cycling process will be greatly extended, up to a month to six weeks has been reported regularly, or if no bacteria are added the process may be longer still. A large inoculation should completely cycle the tank in two weeks with a few days leeway on either side. The second lesson is that long development time for the first step of the process, ammonia oxidized to nitrite, can damage the bacteria needed for the second step due to starvation. That effect will be discussed later.

Where to get the bacteria?

So, where do we get such bacteria? The first and best source is from an established biofilter in a healthy freshwater tank. But if you are a real novice and this is your first tank, you have an insurmountable obstacle there – no other fish tanks, and you may not know anyone with such. Most LFS (local fish shops) will not sell or give you media from their tanks (a combination of unfamiliarity and a missed sales chance, plus potential liability issues). So what do you do? To me the best technique is one used by Chris in his experiments and by most of the beta-testers – watch the LFS or just ask when they get their live plant shipments in, especially the potted ones. These tend to be small plastic pots with rock wool around the plant roots [4].

These plants are greenhouse grown, commonly as immersed culture (the pots are in circulating hydroponic nutrient solution with the plants out of the solution in very humid to constantly misted air). The rock wool in these pots tends to develop good colonies of nitrification bacteria while the plants are getting established and growing to sale size. So, all you have to do is go to the LFS the day the plants come in and by one or more newly received potted plants, and you have your culture.

In my estimation, one pot is enough for a 38-76 Litres (10-20 US G.) tank. Larger tanks might need two pots or require a few days to a week longer to establish a competent biofilter.

If you are using a HOB or canister filter, clip the pot apart gently and tease the rock wool away from the roots. Place the rock wool on a mesh bag or just place it upstream of the biofilter-to-be in your new filter. You want the water to flow through the rock wool before it flows through your new filter pad/sponge/media of whatever sort. I prefer the mesh bag technique, as I don't want loose rock wool fibres forever in the filter.

  • Do note: The plant here is trivial. You could just sit the potted plant in the tank as is, preferably directly in front of and very near to the filter intake, and that will work also, if slightly slower.

The variant given above is just the most effective that I have found. If you plan on live plants, do feel free to plant the new specimen in your tank, but do minimize the lighting during cycling to avoid ammonia-promoted algae. If you don’t plan on live plants, consider that you paid for a bacterial culture and compost or discard the plant.

On the second take-home lesson mentioned earlier, if your ammonia to nitrite period was longer than ten days or so, it would be good to reinoculate with more bacteria. If the second group (nitrite to nitrate bacteria) is without energy food for too long, many will have died. Establishing a functional colony will require even longer than did the first (you are starting with fewer bacteria). A fresh inoculation can cut short that time. If your ammonia-to-nitrite phase is no longer than 6–10 days, you should still have viable bacteria for the second phase and not require a second inoculation. If that first phase is greater than 10–14 days, a new inoculation will help significantly.

These extra tips will, with luck, not be needed in your fishless cycling. But they may help avoid the commonest pitfalls we have seen with the technique.

Article by Robert T. Ricketts, a.k.a. RTR. Layout and wikified by Stuart Halliday.


  1. Footnotes: KH is reported either as degrees or ppm (parts per million). In the USA, one degree is the equivalent of 17.86 ppm CaCO3. In other countries, other standards may be used (often 10.0 ppm CaO per degree), so take care if discussing such on a multinational forum.
  2. Sodium bicarbonate, NaHCO3, baking soda (not the same as baking powder), is a standard and very cheap grocery store item. Arm & Hammer brand is widely sold and available.
  3. Nitrification bacteria are common in nature, even in aquatic environments. But not all of these will successfully establish indefinite lifespan colonies in freshwater tanks.
  4. Florida Aquatic Nurseries is a major brand, and the commonest seen here in the Mid-Atlantic coastal area. This is the brand that I used in my own tests. Tropica potted plants are more common in Europe.