hydrogen peroxide is a strong oxidizing agent, your tank being about 40 gallons or less, you could use 200 ml with no problem, it will stave off most bacterial and fungal infections. It dissipates in the water shortly after.
I learned this from a gold fish importer, as kois and goldfish get antibiotic resistance external diseases, he would pour high concentration of peroxide, with salt, until the fish start to gasp for air, then do a 50% water change until they breath normal. what it does, it burns off all external pathogens in a quick burst. then use copper, and contrary to what people say, almost all fish are ok with copper, except for invertebrates. copper takes longer to work, you will not see it do anything until a week later, but it will kill off most parasites that could cause skin lesions.
I tried this combination with some tiger fish, hydrocynus sp. that had this tail rot, with cotton like infection. it spread very fast, killing one, and infecting another the day later. and after losing the 2nd one, I knew it wasn't from fighting, so before leaving for work, I poured the peroxide in there, and large amount of salt. when I got home in the evening, all signs of infection was gone. and they are still living well today