As soon as you notice the belly of one of your fish beginning to swell, it's time to start making plans. Sometimes you will only have a couple of days before the birth. Make sure you have read up properly on the fish species in question - a few species of fish do care for their young, in which case separating them can be a very bad idea.
Feeding Baby Fish
Many tropical fish hatch with yolk sacks still attached - round bulges underneath their bodies - and will digest these during their first few days of life, as they learn to find other food. For this reason you shouldn't worry if, at first, they seem to get smaller.