I'm doing a 90% WC every darned day in that turtle tank. Ugh! The filter in it is way inadequate. I've got a canister filter coming on Tuesday, thank goodness. For now, I've got the minnow in a QT tank. At least he's getting fattened up in clean water without two dinosaurs wanting to eat him, even if the poor guy is all alone.
Unfortunately, I don't know anyone locally who keeps fish that I could give him to. Once the rocks (his hiding spot) are gone, if he's not history ASAP, I think I'll just give a quick, clean death myself and then feed him to the turts.