[Wantsome99]

I'm trying to figure out why my rainbow fish are dying. Everything else in the tank is doing fine. No signs of illness other then really long white poop about the length of the fish. I'm feeding mysis flakes and freeze dried brine shrimp. Maybe more veggies? Thinking about dosing general cure for internal worms but I'm not sure.

 

[Quagulator]

Praecox Rainbows are notorious for dying randomly. Most stock is from poor breeding / genetics and they just wither away and die for no apparent reason.

 

[Plinkploop]

I wouldn't start adding anything until you've ruled out water quality issues. Do you know your parameters?

 

[Discusluv]

If your water quality is fine, the long stringy poo is a really good indication of parasites. General Cure ( a combo of praziquantel and metronidazole) is relatively mild in terms of medications for treating fish for parasites.
Are the rainbows that are being affected still eating? If so, treat the General Cure in food with a binder. The binder will help the medication adhere to the food long enough for fish to take up medication. Can use Seachem Focus with General Cure or just use a food-grade gelatine like Knox. Just follow the directions on the back of the box.