Flatworms. If they die all at once will nuke your tank. Start siphoning those bad boys out, as many as you can as often as you can. You have a real infestation going there.
Read up on Salifert Flatworm Exit. Read up good if you intend to use it and follow the instuctions to the letter.
Again a mass dieoff or them little suckers will absolutely kill your tank.