I, my tank has been set up over a year with no problems but I have noticed since I done a 40% water change 2 days ago and since then the tank has looked really unclean, fluffy stuff floating around, water not looking clear so I performed a water test and ammonia is 0ppm nitrite is 0.50 and nitrate is 20ppm. They are always 0.0 or very slightly over but I have never had this before, can anyone help?