I noticed a little brown spot on of of my plastic plants about a month ago...now they have reproduced! There are probably about 20 1/4" brown spots on plastic plants, ceramic glazed ornaments, and I even saw a couple on the back glass wall. The tank is a cycled 10 gallon with only two tetras in it. Water params are always perfect and I have not added anything. The tank does not have any lights on it, and it has been relying on natural sunlight for the past year, and its been in pretty much the same spot as always.

What could this be? Brown algea? What exactly IS brown algea?

Yea It is brown algae. I started getting brown algae in my tank just after it cycled. It first looked like round brownish-purplish spots and they steadily increased in size. It eventually covered my ornaments, my walls, my gravel, and my plants.

Brown algea is not actually algae but diatoms, little micro organisms. As far as I understand there are three common reasons for why diatoms might start growing in the tank; it is newly cycled and the water quality is still fluctuating a little, it is an established tank and the water quality suddenly changed for an unknown reason, or it is not getting enough lighting. So your first concern would be making sure your water quality hasn't changed without you knowing, if it hasn't then there is no danger to the tank or your fish. They will not harm anything; my goldfish actually likes eating it off the walls and plants. They seem to have a lifespan where for a about a month or so they grow like crazy and then they will slow down. Mine has not gone away completely yet.

Your next question will be "how do I get rid of it?". I don't know of any chemical treatments for brown algae. In general you just scrub it off when you find it ugly. If you have a gravel vac you can use that to get if off the gravel, and one of those long handled glass scrubbers comes in handy too. My brown algae growth actually does a great job of reminding me when I need to clean my tank. Another solution for you would be to try getting better lights... but I don't know from experience how effective that is.

Good luck!

Oops I forgot about this thread!

Well, water quality is not a problem, and since the tank does not have any lights, I'm sure that is. I recently moved it onto a stand that is across my bedroom from the open window, and now it grows like crazy! I plan to buy a nice hood and lights soon when I've ot money, so I guess I'm scrubbing til then I have brown gravel thankfully, so I only need to scrub off of ornaments and the back wall. ooo question: When you scrub it, if peies fall back into the tank will it be counterproductive?

I scrub the insede of the tank during my water change and I suck up the pieces with the suction cleaner. I had bad brown algae on my plastic plants too and I would take them out occasionally and hand rinse them.

there is one other cure to this funny problem,

first I would like to state is that brown algae isnt harmful to the fish and its more of an uglness to us...

With my tanks I use a Bristlenose pleco to take care of this problem they seem to like to eat it and will keep it down to a min.....

Also this is a sign of either high nitrates or high amounts of phosphates. this can either been cured by water changes or you can get what is called phosorb most LFS will have it or you can get it online

Hope this helps