I tried adding four new banded killies to my established school of 8 the other day, it did not go good. 2 vanished without a trace, one is sitting below the surface and looking pale, and one is dead and being shredded and devoured by the rest.

The new bandeds were a bit smaller than the established ones and much thinner, they got bullied since day one and the banded did not relent as usual. Feeding didn't help, the established bandeds agressive style of feeding scared the new ones, and panic led to the established ones being hunted by the new ones.

I only see one alive (one is dead and being eaten, two vanished).

Why weren't the new banded killies accepted by the established school? Why did they kill and eat them? why do they eat fish too big to be bite sized? (they are picking apart a corpse as I speak). Is this reaction to members of the same species common in bandeds?

I have no experience with banded killies, but other Fundulus species behave the same way when mature. Outsiders are not tolerated! Sometimes in-group members are also eliminated.

I have seen extremely peaceful fish become extremely aggressive when over crowded, is that a posability?

you really have the worst luck of anyone i know! lol. ive never had that issue. ive added bandeds after and they all schooled together. the bandeds even schooled with my seminole killis.

My experience with Bandeds is that they school just fine until they reach sexual maturity...then, watch out!! My success with keeping about 10 of them in a 20-gallon tank has rested upon the fact that the tank is heavily planted and the fish have lots of places to hide. I haven't added any new fish for a while. The males seem to pick on any other visible fish. Interestingly enough, the males swim around the females like puppies when the females are looking for places to lay their eggs. That's pretty neat to watch!

Brian