how to get rid of flatworms

[roblarss]

I have 14 gal biocube that has a flatworm issue. I have been treating with Exit and it brings the numbers down but after a month or so i am seeing at least a thousand of these things. There will be so many that the rocks will be carpeted red by them.
I treated again 2 days ago in morning and evening but today I can see them coming out to the glass. Do i need to just repeat treating every few days or is there any other options.
I also just noticed them starting to appear in my 120gal...