I used Ivermectin for many years to treat a nasty parasitic worm that used to infect my Long-finned White Cloud Mountain Minnows. I'm still breeding the same family 15 years later and thankfully haven't encountered the worm since. It used to infiltrate occasionally during the summer breeding period when the fish were moved outdoors.
Tetracycline antibiotics, copper sulphate, formaldehyde and whatnot have been "stalwarts" of aquaristics since the '50s but I've refused to use 'em for quite a while now. That Ivermectin was a PITA to get too 'cause I had to convince vets of its efficacy when used for fish.
Nowadays I just apply my diet-and-health knowledge (for humans and other "animals") to 'em and that sorts 'em out. Algae is #1 'cause it's the fruit / veg / grass of the ichthyological world. The industry / hobby generally isn't aware of this, of course, mirroring the ignorance we see in the human-health world.
Anyway, that Ivermectin is heavy shit. I wouldn't touch it with a 10-foot pole now. IIRC it was (still is?) used as sheep dip for many decades.