The leeches I had were definitely parasitic on my fish as I saw one attached to the side of a guppy late at night. They attach, feed, then detach, so you might not know that they are parasitic unless you see one actually feeding. The guppy had a small circular lesion on it's side as well. It was fairly easy to see as it's a light yellow guppy. I never did see any lesions on the red platys that were also in the tank.
My leeches would hang out under the driftwood and if I picked up the wood and turned it over in the tank, they would let go with their mouth sucker and wave around in the water, stretching out, while remaining attached by their tail sucker.
I totally nuked the tank. I couldn't live with them ! I killed some nice plants getting rid of them but I was not taking any chances of moving them to another tank. They are just GROSS! Interesting, but gross!!