Thinking about this situation logically, the brown snot has to have come from somewhere, even if it was one or two tiny brown snots that you couldn't possibly see when they were imported into the system. Add the right kind of nutrients and off they go and you have brown snot EVERYWHERE
Then, kill ALMOST ALL of the little brown snots off by removing light and you have a setup of even more nutrients for the very few brown snots that didn't die to just go ahead an multiply even more
Sounds like a never ending cycle.
Maybe the only way of getting rid of the stuff might be to do 100% water changes to remove nutrients until they're gone. However, that still leaves room for one or two brown snots to still survive and eventually snot up the whole tank at some point down the line
GADS
