I've just recovered from a planaria outbreak on my 55G...and it seemed to come from some frozen cichlid mix I was feeding. I've read on here, that the worms can stay dormant in frozen food, and this is what happened to me. I only feed live food once or twice a week in my 55G, and after not having a clue what was causing it, I was extremely frustrated. So I decided to feed my 37G tank with the the frozen food, which I never do, and the next day taht tank had worms. It took 2-3 good gravel cleanings/water changes to get rid of them.