i have a 2 months old tank that i think had been well cycled. the fish have been there that long. what could be the cause of their death , i changed about 20% of water yesterday, everything seems to be fine since then not until i came back home 2-3 hours today and found them all lying down the bottom with their mouths open wide. the water became very cloudy to.there's one survivor though, a yellow damsel that seems not having any problem until this moment. What could have caused this awful thing to my fish?help anyone:frustrat: