Yeah..i've found that if the disc is really gunked up with algae, the H2O2 doesn't do much. That's when I break out the bleach. I've been using the H2O2 weekly for the past few weeks. It seems to be maintaining the disc pretty well. They are all still white and churning out the micro bubbles.

Are you removing it during cleaning? I don't have a valve on my setups at this time.