There are several types of flatworm, the most common is the brownish yellow ones with red spots, there are also clear ones, said to eat the red ones, and Im sure they come in a variety of other colors. Ive had them in plague proportions in the past, and the best way I found to eliminate them was to siphon them from the tank often, I tried the nudi, he ate them, but I would have needed a dozen to decimate the population, my 6-line eats them, but hes in another tank
and theres no way Im catching it. After a few months the flat worms just seemed to disapear, I still have a few, but the population stays very low. Maybe there has to be a certain thing present to make them so prolific, and when its used up they disapear. HTH