I have not experienced that in my tank, this is one reason numerous responses are good. When adding fish, I dont worry about size too much, unless it is a huge difference. I toss the new guy in and watch what happens. If there is aggression towards him I will change up the decore and do a water change to rattle them a little. This has worked for me so far and I my pygos range from 4.5" to 8"Raptor said:Fish of equal size will bicker for domanance. If all your fish are the same size. Not too much of a prob. But if you have 7 inch a 4 inch and a 3 inch. Addind one the same size as one that you already have could be a little fiesty. P's that decend in order are ok. Disputes are quick. The smaller one will back down. And the bigger one will pay no mind. Two the same size will sometimes result in a domanance role. And alot of fighting.