I have 4 fish about 2-3 inch long in a 40L tank and a good filter and is cleaned every 1 - 2 weeks depending on the muck, but my Black moor has a problem, sometimes he sinks and cant swim for long but he also has a white bit on his anus and it looks like theres stuff inside there but sometimes little white threads come out however they do not move and therefore im not sure there worms please help im really concerned