I guarantee you that the people gullible enough to fall for the ivermectin lies cannot math out the dose correctly. And yes, the primary problem is that most of these people don't have parasites (yet, give the raw milk some time) that would actually see any benefit from the drug.

The fish stuff was hydroxychloroquine which is typically used as an immunomodulator for diseases like Systemic Lupus Erythematosus (not an antibiotic), and that's a medication that has to be dosed very carefully with regular screening labs to ensure that patients don't end up with organ damage. The stuff for aquariums is in a vastly different concentration (and commonly mixed with all kinds of other stuff), and again, would be profoundly difficult to dose appropriately.

Interestingly, there is some physiologic basis on which hydroxychloroquine may have helped with the original COVID-19 strains. The original strains of the virus early in the pandemic had the ability to hijack immune cells that would then attack and destroy lung tissue, so an autoimmune treatment might have had some benefit had the virus not mutated so quickly.