is a parasite spreading?

Hi.

I've had a few fish losses that I chalked up to stress from the move
(Otos, pygmy corys). Unfortunately, I'm starting to think that I may have
a more serious problem developing. I've seen a few of my fish who look
like they are scratching themselves on the substrate & plants, which I
have heard is a symptom of parasites.

I have aquari-sol, and I put a dose of that in last night. I have started
getting water ready to do a 30-40% water change, but I'm thinking that I
should consider medicating my tank. Any suggestions about what I should
try? Other than the scratching, I haven't seen any other symptoms yet --
no ich, no fungus.

Should I do a major gravel vac, that is, remove all of the driftwood & big
rocks and gravel vac all of the hard to reach places? I'm torn between
worrying that the additional stress of mucking around with all of their
hiding places / territories vs. missing some debris that might be
contributing to any developing illness.

Any advice?

Thanks.

--chris