With the change over it probably went through a mini cycle which leads to greater numbers of diatoms and such, which in turns feeds the copepods and their numbers climb who in turn yet again, feed the flatworms. Eventually, the diatoms dwindle, the pods dwindle and then the flatworms dwindle.... usually.
Yours look like a species of Convolutriloba. Their numbers can definitely climb when they're fed well. Problem is, they can also use your light as energy once the food is gone. That's where the clear ones (Ghost Flatworms) are at a disadvantage. Once the food is gone, there's no other energy source for the Ghosts to turn to, so they die off. Their numbers don't climb because they are usually limited by the numbers of pods.
As far as advice on the Flatworm Exit... I don't have any. It's not something I would use personally.
