can someone help use please we have had our fish for just over 2 weeks know, we change the water every week (20%). we feed them about 3 times a day. we did notice that one of our moolies top fin was going down a bit , but he seemed happy enough. we have 2 moolies. funny thing was that one was getting fatter and the other one was'nt. this morning my wife found him and he had died. we need help as we love our little fish and we don't want any more too die. are we doing anything wrong.