Water change

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A regular water change regimen is the most important maintenance activity for both fresh water and marine aquariums. This involves removing and discarding some of the tank's water, and replacing it with appropriately conditioned clean water.

Minimum recommendations are 10% a week for fresh water or brackish tanks, and 25% monthly for marine aquaria. More is generally better.


The minimum level of conditioning is to allow the replacement water time to reach room temperature, or, ideally, be heated to tank temperature. Usually this time will allow for any chlorine to evaporate from municipal water, however, if chloramine is used locally, it must be removed chemically.

For marine tanks, usually dried salts are mixed with clean fresh conditioned water. A few days or more will allow the salt mix to fully "integrate" in the solution. Some people have access to trusted clean seawater sources (which are highly recommended), and also some pet stores sell "ocean water" by the gallon for less than the price of gasoline or milk.

Basic process[edit]

The process starts with the last step, in that it is an ongoing, repeated regimen. This is described for fresh water tanks, marine tanks are similar but more expensive and scary.

  • Carefully siphon the desired amount of water from the tank (10-20%) and discard. During this process, excess mulm may be removed carefully from the substrate surface, or even below with various large diameter suction tubes.
  • While the water level is low is a good time for any planned redecorating, replanting, etc. chores. It is a very good time to scrub the front of the tank to refresh your algae-obscured view.
  • Treat the new conditioned water in whatever manner is standard for your tank - pH buffers, plant growth formulae, etc.
  • Carefully add the new water to the tank - the nicest way is to place the container of new water on a tall stand of some sort and siphon it down, gradually, to your tank. The next easiest is to pour it carefully over an area of the tank that will not be much disturbed by the water flow, such as a piece of driftwood or large rock. It can also be carefully poured into a power filter's rear compartment, allowing the new water to be processed slightly by the filter's media. If you are running external canister filters, a bit of ingenuity can be used to get them to suck most of the new water into the tank via their intake tubes.
  • If you haven't heated the replacement water, be sure to note its effect on the tank temperature. A drop of 2 or 3 degrees F (1-2 degrees C or Kelvins) is not a big deal, especially if your heater brings things back to "normal" within an hour or two. If you notice more of a drop, either try to increase the temperature of the new water, or add it more gradually.
  • Wipe off the front of the tank if you spilled any water on it.
  • Observe your tank for a bit to make sure everything seems happy. This is part of the fun.
  • Finally, as referred to above, prepare your next supply of replacement water, so it will have plenty of time to warm up, and will be available on a moment's notice if you want to do more frequent changes.

Finally finally, with all this work complete, once again, sit down and relax and observe your aquatic companions for a minimum of one hour. Feel free to cite this page if this conflicts with spousal duties.

Why change water?[edit]

Over time, your aquarium water contains more and more dissolved waste products, mostly nitrates, but also any other chemical that may be present in "make up" water used to compensate for evaporation. Periodically removing some of the water and replacing it it with clean water reduces the concentration of these chemicals.

Also, regular water changes make it easier to maintain a "baseline" chemistry - a tank can run for years (especially freshwater) with minimal water changes, but its chemistry will slowly change. The long-term occupants might be able to adapt along the way, but any new life added will not be able to cope. Regular water changes make it easier to keep the tank in shipshape order.

See also[edit]