I would tend to think that the filter media that is suspended above the water line could be a culprit, but you say you maintain it once a week....so thats most likely out.

You say you used an older (but not old) sand bed from another tank, and added new sand. Not sure if this could be a source...thinking its not.

11 fish, 2 Hippo tangs, a Powder Blue Tang, a Yellow tang, 3 Green Chromis, Lawn Mower Blenny, Cardinal and 2 Percula Clowns, in a 90 gallon, that are well fed, but not over fed....Hmmmm......maybe we found the source. Its very possible. I would say nitrates at 20ppm in a tank heavily stocked isnt really all that bad. Tangs are VERY messy!

Im going to think on this one a bit more.....

Thanks for such a thorough description of your tank! It will help alot!