I've lost 2 out of 7 mollie fish in the last 3 days after having them thrive for 2 months. I've noticed a sudden brown slime covering the gravel on the bottom of the tank and the artificial plants. Also, I've noticed white spots on the 2 fish that have died and one of the remaing 5 fish. The white spots I have identifed as most likely itch, but why is everything in the tank turning brown? Any suggestions.