Just in this past week, I have noticed changes in at least a few fish in every one of my four tanks. Three of the four tanks have only mollies, while the other tank has wild and tank-bred mollies, angels, and some rainwater killies. The mollies seem to be the only fish affected. In one tank stocked with only mollies, I did an 80% or so water change (per the usual, so nothing out of the ordinary) and about half of the fish in the tank started to 'shimmy' and become very lethargic, clamping their fins against their bodies. The next morning, half of the fish had passed. In the few days since then, there are three fish left in the tank, and all are shimmying with clamped fins. In the other three tanks, no fish have been lost as of yet, and only a few fish in each tank visibly appear to be shimmying. Most fish are swimming with clamped fins, and a few are even appear emaciated. Though, I see them eat every day, just as much as they should. All tanks are moderately planted, 79-82 F, nitrates never greater than 40-50, and 0 ammonia and nitrites. I haven't dealt with anything like this in quite some time, and after doing lots of research, I was thinking the culprit may be some type(s) of internal parasite(s)? I'm planning to treat with Prazi-Pro, but the real head-scratcher is what caused the mollies in all of my tanks to become stressed to the point of being susceptible to an illness, all within one week?