This should answer you question:

Brown Algae - Diatoms

Diatoms are single celled algae, brownish in color. It starts out as a
dusting on the substrate that rapidly turns into a mat that can cover
the tank over in just 5 days, provided that they have a surplus on
nutrients available.

The cause: excess nutrients, silicate and silicic acid are the
ingredients for a diatom bloom. Silicic acid enters the aquarium by
using tap water; substrate can contain silicates that leach out over
time. RO/DI units can filter silicic acid out, but only for a limited
time. Eliminating silicates can control diatoms.

HTH

Tom L.L.


(Magic menagerie) wrote in message ...
I hear green algae is ok, though too much isn't. Our tank seems to have too
much brown gunk/algae. I scrubbed it off three days ago, and now it's back...
What is it?