As an unintended consequense of using various liquid phosphate removers (assumed to contain lanthanum chloride as a “propriatary” ingrediant) I noticed flatworm die off. I have witnessed this in different tanks and using different brands. The die off is fairly slow but usuall within 2 or 3 weeks they are gone. I don’t overdose the po4 remover, if anything less than what is recommended. I have no idea why this happens. Has anyone else had this occurance? The remaining flatworms are definately still alive, just less active and darker in color. The skimmer is pulling out smelly dark stuff, toxins I suppose. Running carbon also. Its happened in 3 tanks using 3 different brand removers. Others thoughts?
Perhaps the lower P04 is killing off algae which in turn maybe killing off some of their food sources?
Skimmer snog being pulled is likely from the higher die off of said food source and flatworms.
My guesses!
You’‘d likely not notice any die off if there weren’'t flatworms in there to begin with! 
(I joke)