Tips for fish-in cycling your freshwater aquarium

Tips for fish-in cycling your freshwater aquarium

  • Author Ouse
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Fish-in cycling is not recommended for any hobbyist if there is an option of cycling methods. It is strongly advised that all hobbyists use the fishless cycle rather than cycling with fish. However there are some circumstances that can give a fishkeeper no other choice than to use a fish-in cycle, such as misinformation being provided and parameter spikes. Even if a tank has previously cycled a fish-in cycle period can occur in established tanks as well as new ones.

To successfully finish a fish-in cycle in a freshwater aquarium you want to start off by keeping as little fish as possible in the tank while still having some fish. Fish that produce little waste but enough to produce sufficient ammonia supply to the bacteria are ideal such as guppies, platies and bettas (the latter surviving fish-in cycles very often due to their air breathing adaptation). To limit the waste the fish produce feed them small but sufficient amounts of food. Invertebrates will contribute very little to the bioload of an aquarium, and while in most cases this is beneficial, it won’t keep the bacteria fed throughout the cycle, so use fish instead. To keep the fish you are using alive and to allow the bacteria to overpower the ammonia supply you want to do 30-50% water changes once the ammonia and nitrite add up to 1ppm while using a water conditioner/dechlorinator on the new water before adding it to the tank. Seachem Prime is recommended because it dechlorinates tap water and binds ammonia into a non-toxic form for 24 hours. The only downside to Prime is that it can create false ammonia readings when testing the water.

Speaking of testing the water, you want to get a drip test kit for the cycling period to check the progress of the tank constantly. Avoid using test strips at all costs because there are very little occurrences where they give correct readings. Conduct your drip tests in the light, preferably by a sunlit window to get accurate and easy to read results.

The six parameters you must look out for in your aquarium are ammonia, nitrite, nitrate, pH, carbonate hardness and general hardness. Ammonia, nitrite and pH are the most important to watch because they can affect the performance of the fish and the cycle. Ammonia and nitrite needs to kept preferably at 0ppm (parts per million) and the pH equilibrium varies depending on the fish you are keeping. For cycling the pH level is 7-8 ideally, and pH levels that are too high or too low will affect the speed of which bacteria can multiply in the aquarium and the acidity/alkalinity of the water. By doing daily water changes as stated before you can help keep all the parameters closer to their equilibrium, keeping the fish, bacteria and water healthy.

Thanks to the bacteria ammonia is converted into nitrite, which is then converted into nitrate. Nitrate is best at levels of 0-20ppm while the other two are best at 0ppm. Achieving 0ppm for nitrate is near impossible as it‘s always being produced in a cycled tank. The reason for doing water changes even after the tank has cycled is nitrate doesn’t get converted into anything by the bacteria and has to be taken out via changing the water before the level becomes too high. A nitrate level of 100ppm is enough to kill fish however it’s best to keep it far below this level by doing weekly water changes.

In established tanks fish-in cycles can occur but are very short-lived because the bacteria colony is already developed and can combat ammonia spikes much quicker. Once done there will be a higher than usual level of nitrate as a byproduct of the nitrogen cycle but with water changes the parameters can be kept at a safe, manageable level and the cycle should take even less time. In an established tank removing fish from the tank shouldn’t be necessary as long as you’re intervening with the cycle sensibly. This once happened to me as my tank’s bioload increased heavily but the cycle was shortly over as I did 40% water changes when parameters became higher than wanted.

So there you have it: tips on how to succeed in fish-in cycling your freshwater aquarium, should you need to use this method. Fish-in cycling should be reserved only for emergencies and not used on a new tank.


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