I was wondering if somone would give me some advice..

I have 2 turtles, a yellow bellied slider and a western painted. The slider was about 3" long when I got him.. he's around 5-6" now and the painted is about 3" long. They're together in a 55g and did fine for weeks.. until recently. The slider started to try to bite the painted's tail! He tucks it in but the slider ended up biting the shell above his tail and a pretty decent size chunk is gone. That area of the shell seems pretty soft too and I don't know why.

I realize separation will obviously stop but that isn't the easiest thing for me, i have nowhere to put him.

The slider eats and eats, is he so hungry all the time that he's trying to get some extra food? Other than the nipping, he doesn't seem aggressive towards the little guy.

The slider eats 2 different kinds of turtle pellets, minnows, & krill.. he ends up eating whatever I feed the few goldfish too. The painted will only eat minnows and krill.

How serious is this? I'm getting nervous/ don't want to get rid of one but I will if I have to.