There are some fish that may help "control" a population, but if it is a full blown infestation/invasion, then you will need to do both manual removal (or possibly chemical warfare) and then add a fish to keep the numbers in check.
Some potential flatworm eaters are:
SixLine Wrasse
Yellow Coris Wrasse
Mandarin Dragonette
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32"x32"x20" Cube-ish tank
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