Join Date: Jul 2011
Location: Fort Myers, Floriduh
Almost any filter that has been running for a while has enough of a buildup of organic debris in the cracks and crannies of the filter to support the needs of the nitrifying bacteria for at least a short time. One thing to keep in mind is that nitrifying bacteria colonies grow in direct proportion to the food source (organic waste) that is available to them. I think that how you do it depends on how big the bioload is; in other words, how many fish are there. If the tank is understocked, then it might be safe to move them all at once, depending on the individual needs of each fish. Tetras, for instance, are typically very hardy and can stand ammonia/nitrite/nitrate levels that, say, otocinclus catfish could not. If you are not sure, and you have the time and patience, err on the side of caution and move them slowly, as bsmith suggested, and you will probably be just fine.
I even have duckweed in my water bong!