iwagumi maintenance?

using an algae pad to scrub the top of the rocks works well see: http://www.youtube.com/watch?v=nbZqDPmuWFA

alternatively you can drain the water level down below the surface of the rocks and with the rocks exposed above the water line, spray gently with a hydrogen peroxide / water mix solution from a spray bottle to make the algae fizzle away.

Old tooth brush, something simple and there are always a bunch of used ones on hand.
I even make wood extensions for them.
md