I had the same problem in the beginning with my tank 36g, and eventually it went away try using polyfilter pads in your filter, I am now dealing with red slime which i finally figured out is from high phosphates so now am using phos-zorb insert and phos-guard beads with filter bags and as of today (did a phosphate test all is ok) ....but def from high phos and silicates....(tap water problems usually and some home under the sink filters dont take out phosphates ) But also am using Purple up now to help speed up benefitial coraline, so far so good except that it clouds the water for like 2 hrs+ after dosing.......