Umm, this might be a dumb question but do you have a reliable test kit? I had this problem with my bros 135. I did 2 50% water changes in 3 days and pulled half the rock out and scrubbed it in the first water change, and pulled the second half out and scrubbed it on the second one. The prefilter sponge in the overflow got cleaned, the polyfilter in the wet/dry got changed and a lot of carbon was added. I added enough bacteria to treat 500 gallons since there was probably quite a bit of loss there. (As scary as it is we had to clean the bioballs as well.) Now we just keep on top of it, scrubbing the rocks when it forms again, and doing water changes monthly. The tank is a FO right now so it is probably somewhat different from your situation, but I wanted to share that with you in case you get so fed up with it you want to wage war on it (that's what I did, and so far I'm winning).