I have a 60 gallon tank with 12 tetras 1 molly 5 platies, 6 guppies, 4 albino corys and 2 plecos . I just did a 50% water change, vacuumed the gravel and changed 1 of the 2 filters. Since then all my fish are dieing or acting sick, they are hanging out on the top or the bottom of the tank. I did...