I would say foe flat worms can turn into plague numbers, syphon them out. I would suggest something to eat them but when the food supply is gone they will be to.
I do not know the actual scientific name, I want to say its red or pink planaria? Some others better with idents may post today and help me out on that one though.
Use "flat worm exit" if they are in small numbers, other wise syphon them out to thin the numbers FIRST. Other wise the sudden death will increase the amount of toxins in the water they exude when they die to leathal amounts and cause a tank crash.