keeping your water cool? if warmer, becomes a breeding ground.
a little staining is no problem, but slime means bacteria etc. either use h2o2 or some sort of beneficial microbs to compete for resources. 1 or the other, not both.
try finding a volume that allows for frequent enough topping off to keep it fresher longer? i find topping off at 50% is better than letting it draw down near minimum as far as how long before i need to clean it.