You know, Ive been fighting this same SHit in my res for the last 2 months. Slowed it down by completely blocking light out, not using hardly any ph up or down, i read that could be the cause, and keeping res temp at about 68,69F. The slime stunted root growth a little, but I cleaned the res out with bleach when i did a nute change, and finally was able to get a handle on it. Basicly, i think it might be something with the gh nutes cause ive used others, and never had this prob. my thoughts....