Furan 2 is avertised as being good for the treatment of gram negative and gram positive bacteria. Since all bacteria that are not gram positive must be gram negative I find it difficult to see how it will have no effect on the bacteria in the filter. Some fish are I understand sensitive to furan 2. Methylene blue is a dye and useful in treating fungal disease and it stains (dyes do that) Malachite green is a dye and is useful in the treatment of fungus, bacteria and parasites like white spot but some fish are sesitive to it and it is a poison. Acriflavine is used mainly as a bacteriacide. Quinine is used to treat parasites like white spot and it is a restricted poison (can cause abortion I understand). So you pays your money and you takes your pick. Any drug which kills bacteria, plants , fungi or parasites is not likely to be much use unless it is a poison. Treat your fish with it and not yourself or your kids. Having said that quinine was used to treat malaria, meth blue to treat athletes foot and acraflavine to use on cuts and abrasions.