not to insult you but your tank is dirty excess food and nutrients wil cause an explosion of flatworms all tanks have them but they only become a problem when the above occures one thing that is often overlooked is excreament from the animals you are feeding if you overfeed then their feces will be very rich with food as it has a tendency to pass thru them quickly when food is abundent this is a trick that nature uses so the animal is able to eat a lot extract what it needs and dump the rest when food is scarce then the organisims retain the food longer in their gut there by lowering the nutrient value of the feces.
Just a question out of curiosity. When I have observed these flatworms in tanks, they are always in the light. Has anyone determined if they indeed have some sort of photosynthetic symbiosis? I wonder how much of their nutrition comes from detritus etc and how much from symbiosis.
There are definitely flatworm (Platyhelminthes) species that have dinoflagellates living in their tissues, I was just wondering if the plague type brown-red form I have seen in reef tanks does. The flatworms I have often observed do not shun the light, rather they stay in it. Perhaps we are talking about two different species.