Interrupted Nitrogen Cycle
If your aquarium has been sitting empty, it may still have some beneficial bacteria.
Q. I have a question concerning aquariums in which the nitrogen cycle has already been established. I recently populated a new 20-gallon "long" aquarium with several gouramis in order to set the nitrogen cycle in motion. Several weeks later, after the ammonia dropped to undetectable levels, I removed the fish in anticipation of adding a large school of tetras. However, I was interrupted by other things, and the vacant aquarium continued to sit there with the filter and heater running. A month passed before I was ready to add my tetra shoal. Did I wait too long? How long does it take before the nitrogen cycle comes to a halt? How long can ammonia-converting bacteria survive without nourishment? Should I begin by adding only a few fish once again?
A. It depends. A couple of weeks isn't a big deal, but a month is long enough that you should play it safe and introduce fish in batches. If your aquarium is heavily planted, I'd go ahead and add all the fish. Live aquatic plants can use ammonia directly from the water. Indeed, heavily planted aquariums usually show no ammonia or nitrite spikes at all when first set up - even with a full load of fish. Having live plants also means it's likely that bits of vegetative debris and hitchhiking snails have continued to provide some food for the nitrifying bacteria while the aquarium was vacant of fish.