Cyanobacteria- how to get rid of it

More light will NOT stop it but rather increase its growth. I used to battle algae a lot too. You might have phosphates in your water which are promoting growth. You can use phosban to suck out phosphates or you can use something more natural. I used chaetomorpha (sp?) which competed with the algae for nutrients. I think its sometimes called brillo pad algae. (green, looks like a brillo pad, duh) My emerald crab used to like it too.

Are you using tap water for water changes and top offs? I tested my tap water for phosphates once and they were off the charts. For top off's I buy those 2.5 gallon jugs of water. You can use RO water also.

Are you over-feeding? Extra nutrients in the water might help algae grow.

How is your hardness? I'm not sure how true it is, but I was told that if your hardness is around 11 or so algae has a hard time living. I keep mine right up there but not sure if thats the reason why I don't have much algae.

Is your tank by a window? Getting lots of light will only stimulate algae growth.

I am sure there are a bunch more reasons for alage growth but these are the main ones. I would take your water to your LFS and have them test for phosphates before spending the $$ on phosban. If they are just slightly high, you might try that brillo pad algae. Just be carefull buying it. Some people sell caulerpa, which can "root" itself through out your tank and is often undesirable. I am quite sure the brillo pad algae will NOT do this. Mine never did.

Ok good luck.
~d

I had a minor outbreak as my tank started maturing and found that keeping the tank as clean as possible, increasing circulation and getting PO4 and NO3 as close to zero as possible really helped. I wouldn't resort to chemical means of dealing with it unless you absolutely have to. Not much eats cyanobacteria TMK and limiting light doesn't have much of an effect AFAIK.

I discussed this problem with the owner of a 500g reeftank. His advice was circulation. Circulation is the answer. The flow of your powerheads or circulation should exceed the tank volume by 20 to preferably 30 times and not allow dead spots.
Example, my 25g has a powerhead rated at 400gph. This gives me 16x my tank capacity in flow per hour. My skimmer pump is rated at 200gph which gives me another 4x tank volume flow per hour, with my ac filter, I have another 4x tank volume flow per hour. So, the total flow is 24x my tank volume per hour. Also, make sure there are no dead spots (where there is no flow at all) Arrange your rock so there are spaces where water can flow thru, avoid solid walls. DO regular water changes at 10 to 20% with r/o water if you have a skimmer, and 40 to 50% if you don't.
This worked for me.
Cyano is not like green algae, dependent on light. Length of lighing time has no effect on it, it will grow under less light, unlike green algae.

Its amazing how fast it clears up once you get the water moving over it.