I noticed that some of my freshwater fish have redness around their gills. It's a 29 gallon with mollies, cories, assassin snails, and a tetra (the others kept dying within a day of getting them in the tank.) I have sulfaplex on hand right now. Is that safe for the cories and snails? There are two mollies and a tetra with red gills, and 2 other mollies have been hanging out near the bottom and not really swimming, but not flashing either. I should also add that my water parameters are good, it's a planted tank, and I do weekly water changes of 4 gallons with about a gallon added mid-week due to evaporation.