I think toxin referred here is nitrate. Removing it will not cause nitrate outbreak if done properly. Nonetheless I dun believe will solve the root of the problem. I use bio-ring and rinse it every month to remove whatever stuck there and replace whatever water there, it is okay if well maintained.
Back to the reds, I think it may be true that the LS suffocated whatever those red stuff. For your polyfilter to be that color, the water has high concentration of it. Worst if they absorb oxygen.
One suggestion, is to blast air into the tank, add a skimmer if you have combine with whatever chemical media you have, and one week you will be up and running.