Don't put fish in the garage and rats in the closet. I have no idea how to do water changes on Sparky without shocking him because the hose water is like 65 F. But he's got like 40 ppm nitrates. I have to do a change in the next day or so . . . somehow. And the rats pulled the hem of my brand new dress into the cage and nibbled holes in it. T_T Brand new, dangit!
I must have nitrates in my water. Even my heavily planted, triple-filtered tank gets up to 40ppm in a week.
And honestly, I don't know how this guppy is alive. He has no dorsal fin; the loaches bit it off a long time ago. But he waggles along anyway.
Tiki, Kanaplex will be gill-absorbing but to target an internal infection, it will be most effective being fed.
Mo, that's a lot of fish. Good luck. :)