to the site and to the forum!
Turtles are omnivores, so they eat all sorts of things, including fish. If your turtle were to give you a problem, it would be in the form of fish skeletons. I don't think your turtle is the problem. Our turtles over here (Western Painted) do just fine with the fish in lakes, ponds, and streams (obviously excluding the fact that they will eat each other, with the fish eating the turtles' young, and the turtles eating the smaller fish).
And if the turtle had a disease it was spreading to the fish, the turtle itself would most likely be having problems. I think there's something else going on here. I think cleaning out your filter is your best option right now…if you still have a problem, we'll take try to figure it out from there.
ETA: Is it possible that maybe the fish just died from old age? Although I don't know if fish get tumors when they die from old age…
