ok, so I have a ten-gallon plated betta tank and I moved my betta to a twenty-gallon last week. about three days after that he just stopped eating I thought it was just stress from the move and the high amount of surface agitation from the sponge filter so I moved him back to the ten-gallon. then I noticed that his fins were fraying and that there was some white stuff growing at the base of his tail. I thought it was just fin rot or an external parasite so I was just gonna get some general cure and fin rot meds but the next day he died. At that point, he had not eaten for four days and when I scooped him out of the tank he had white fluffy stuff on him but not in his mouth just on the fins and body. I think it was fungal but have no clue how it got to him as he hasn't had any tank mates besides two nerite snails for about a month. My problem now is how do I make my tanks safe for new fish in the future. I will most likely take down the ten-gallon as im, not too pleased on how it looks but I don't want to dismantle the twenty-gallon. How do I make it safe for new fish? p.s I have no other fish only the two nerites I also don't have pictures of him while he was sick.