IDK where you live in the US, I'm in WI and as far as I know Minocycline HCL and erythromycin are prescription meds. I have prescriptions for both, though the erythromycin is a 2% solution, I've never used it for my self or my fish. I have used the minocycline, which comes in 100mg capsules. Dosing the meds for fish is actually quite easy, I follow the directions for the fish meds Maracyn and Maracyn-Two. Maracyn is erythromycin, and Maracyn two is Minocycline.
The directions for Maracyn are- dose 200mg erythromycin per 10gal every day for 5 days, repeat only once if needed.
Directions for Maracyn- two : dose 20mg Minocycline per 10 gal on the first day. Second through 5th dose 10mg Minocyclin per 10 gal. repeat the 5 day treatment only once if needed.
You can use solutions of both meds to treat fish with, but working out the dosage is more difficult and is impossible to say with out knowing the % solution.
BTW- both of these are prescribed most often as acne meds, so if you can get it OTC where you live look in the acne section.