How warm (or hot) is the water mixture in your reservoir? I know that in my small space my res was heating up and I began to see Brown Slime on my roots. I did the H202 thing which helped a bit, but it came back. I was told by my local indoor garden store to try a product by Dutch Master called ZONE. It cleared up my problem in a matter of days, plus promoted a burst of new root growth. Their website has comprehensive dosing instructions for different severeties of root rot as well as amounts for different temps of nutrient mixtures. With this product a little goes a long way, it was well worth the money. Hope this helps.
Peace