I have a 55 gallon freshwater tank. It is mostly Tetra, Pladdies, Guppies and has 2 larger Guorami fish. I also have 2 swordtails and one has this white spot that looks like a pimple on the base of his tail. I have treated the tank using Maracyn Plus 2, Paraguard, Melafixin, and aquarium salt (not all at the same time). He has had this spot for a while. We recently bought a RH filter and we process our water through that. The water reads a perfect 0 for all chemicals/particles when we change our water now. From my tests today my tank has a PH of 7.15, the ammonia is about .1 ppm, the nitrites are similar and the nitrates tested at 10 ppm. I have a Marineland canister filter and a Marineland over the top bio wheel filter. The swordtail swims and acts normal he just has a bump. I have baby pladdies that are living and growing so I take that as a good sign. Any suggestions as to what the white spot is and how I can get rid of it I would greatly appreciate! Thank you!