Flatworm killers!

[Delphinus]

I say go for the canary wrasse. Very nice fish in their own right, but mine also took care of my flatworms within a few weeks. At least, I started noticing that I didn't have flatworms anymore a few weeks after getting the wrasse, I assume I had the wrasse to thank for that.

They are jumpers though. Make sure you have a cover or something (even then apparently it's no guarantee, mine found a way through eggcrate earlier this summer).