This is an absolute pain! I want to intervene as I don't want them getting eaten. But, if one gets eaten, then the other two will hopefully be more vigilant around the turtle...I mean...its only natural. I'm sure you keep them in check else you wouldn't have that many! I used to have just about as many pets as you do minus the birds and I loved it.
It doesn't look like popeye. It is whote. I have tried getting a picture, and it is really hard. I'll try againIch, or popeye? Quarantine that fish if you're able. Try to quickly narrow the possibilities to determine if it may be caused by a parasite in their environment.