Kordon 2 Pack of Methylene Blue, 4 Fluid Ounces Each, General Disease Prevention for Fresh and Saltwater Fish, Treats Fungus and Fungal Growth on Fish Eggs

Although Kordon is not USP rated, it is used by established longevity groups. So, I trust it. The price is much cheaper than the USP rated brands. I believe it is because they do not feel the need to pay for the rating. Anyway, we've been using it for a couple months and no ill side-effects noted.