When To Add Fish to a Tank

The first consideration when considering more fish is to evaluate the potential capacity of the tank and it's systems to handle the new addition(s)...not just in the beginning, but as the fish grows into it's eventual adult size.

When you are dealing with an established tank, one that has competed a nitrogen cycle, you may want to add fish from time to time. Keep in mind that an overstocked tank is much harder to deal with than an understocked one. The chemistry of a heavily stocked tank can be rather unstable.

Each time a fish is added to a system, there will be a period of ammonia, nitrite and nitrate spiking. If the fish is small, the spikes will also be small, perhaps to the point that most of the common hobby grade test kits would have a difficult time in indicating the increased levels.

If you are wanting to add more than one fish, the combined mass of the fish will have an impact on the tank environment proportional to the mass. The more is not neccesarily the merrier in the case of adding fish. Unfortunately, adding a shoal of Corydoras or a school of Tetras is an appealing prospect. However, adding a number of fish at one time can have a profound impact on the tank. You will need to plan for that impact.

Prior to adding new fish to a tank, the tank should be reading zero for both ammonia and for nitrates. Your water changing schedule should be designed to keep your nitrate levels at a stable number. You should be testing each of these levels weekly prior to adding new fish and for the month following their additions.

When the numbers are zero and in the case of nitrates, stable, you can consider adding new fish. You should plan for added ammonia production into your system and the subsequent requirment of bacterial habitat to mitigate the ammonia, followed by the nitrite. In simple terms...you need to make sure you have enough capacity in your bio-filteration system to handle the new fish. It is always a good idea to have more capacity than you plan to use and always more than the manufacurer suggests.

As you add more fish you will want to continue your testing as suggested earlier. If you add a shoal or a school of fish, don't be surprised by a significant spike in readings but also , don't alter your water changing schedule in response to the ammonia or the nitrite readings as your bacteria colonies in the bio-filteration systems will need those compounds to grow to meet the long term goal of zero readings. Only change your water changing regime in response to changes in your nitrate reading.

A hint:

Not all decisions about adding fish are chemical. Of course you will want to consider the compatibility of fish when you make decisions. We have found that changing the environment (cave shapes and locations for example) will help a when adding fish into a tank of territorial fish (most cichlids come to mind). By changing the environment, the domination of the territories is temporarily halted and allows the group to settle in a new pattern with new players