Hi all. I have a 20 gallon fijicube with chaeto in the refugium (it may be hard to see, but it is in fact in there). I am using a fluval nano light, running opposite the display tank. I am having a problem with a reddish brown buildup on the acrylic walls. It is pretty difficult to scrape off with a scratch pad. I am not sure what it is specifically. I have had issues recently that are being handled with high phosphates (currently at 1 ppm, down from 3 ppm and continuing to drop), and high alk (currently around 14 dkh, and slowly swapping out salt from reef crystals to fritz). My skimmer is finally working properly and pulling phosphates effectively. I know that the photo doesn't really do the issue justice. Any advice?