Serpae Tetras

I'm afraid serpae tetras are well known fin nippers, especially if there aren't enough of them. They need to be in a much bigger group than 4, and even then I wouldn't have them in my tanks  . I had them once, never again. Nasty things.

The easiest way to catch fish is during a water change. The less water there is, the less space there is for them to escape to. Remove any decor that you can, but don't uproot plants. Use two nets; use one to chase them into the other. Or a net to chase them into a glass jar - the theory is they can't see the glass.