guppy with a red spot during med trio treatment

[Adron]

Hi friends! I just started the Co-Op's recommended med trio treatment yesterday in a 20-gallon with a relatively young nitro cycle going in it, but a fair number of plants. This morning I saw a red spot on one of my new guppies' tail, and it looks sort of 3D, like a growth or something hanging onto the tailfin. The internet said "red spots bad! ammonia spike!" and I panicked, about to abort the whole med process and water change the heck out of the tank, but first I tested my water, and ammonia looks to be near zero. Nitrites zero, and nitrates near zero as well. pH and KH however, are quite a bit lower than I'd like - KH is ~ 20 ppm, and pH is ~ 6.2-6.4ish. GH is high; ~100-120 ppm. 

Since it doesn't appear to be an ammonia spike, does my low pH buffering capacity have something to do with it? Or is this red spot a predictable response to the med trio treatment? Like, is it some kind of evidence of parasites leaving the body? Or is that wishful thinking?

I've thrown some API Stress Coat in there to try and raise the vibes in the tank, and I'm trying to take pause before freaking out and aborting the medication mission with water changes. But could use the advice of this wizened community! Thanks so much.

[Adron]

To be a bit more clear what I mean by "near zero," I basically mean "zero." But both my tests (API master kit with test tubes and API quick test strips) require that you read results by matching colors to a chart, and I always find that sort of vague and challenging. But the results came out as close to zero on the color charts as my eyeballs can figure, given that the test doesn't give you total precision. 

I understand even small amounts of ammonia and nitrite can be a problem. My trouble is I need to carefully weigh the pros/cons of messing up the medication process with water changes.

Thanks!