Hey---

My rockwork is about 85% + coraline alge. But about 10% has a rusty colored
alge covering. It scrapes off relatively easily.

I didnt think it was diatoms because when I get diatoms on the gravel it
usually goes away by itself in a few days. This stuff isnt budging. Does
anyone know what it might be and what I can do to get rid of it? I cant
really scrape it off without disturbing the rockwork.

thanks again........