Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Was it a planted tank? Will you keep the plants? They are a big part of the bio filter.
a) Keep the plants (drifting, or in pots), and decor, just remove the substrate: You are probably removing about 25-30% of the bio filtration. The plants can pick up their use of ammonia if other conditions are right (Carbon, light, other nutrients are there). Might be safe to stock with over 50% of the original fish mass.
b) Never was planted, but now you add drifting plants or surface floaters: Very good way to add bio filtration. Surface floaters get their CO2 from the air, so as long as other things are good, they are very good nitrogen sinks. Could probably stock with more than 50% of the original fish mass.
c) Was planted, now you are removing plants. This is the worst. The plants were part of the bio filter, and not so much bacteria was in the tank. If you remove plants and substrate then I would stock only about 25% of the fish mass, or else do a fishless cycle to build up the bacteria population before adding fish.
I would restock immediately, but with only 1/4 to 1/2 the fish mass as was in there before.
Do not wait, the bacteria can starve with no ammonia.
Do not stock slowly, the bacteria will die off to match the bioload.
As a rough guide, the filter holds about half the bacteria, the other half is all over the tank, on the substrate, decor and so on. In a well set up planted tank the plants may be 50% or more of the bio filter. Stocking will depend on not just the bacteria population, but also the planting before and after.
Set up the tank the way you want it, then do a fishless cycle.
It will go really fast with plenty of bacteria in the filter, then you can fully stock the tank with no worries about ammonia or nitrite.