From Steve Hampton's website (plant guru extraordinare) at aquariaplantsdotcom:
BGA/Blue Green Algae/Cyanobacteria
This is actually an organism that is more closely related to bacteria than algae but we always lump it into algae for ease of discussion. BGA has many species and forms and causes great angst among planted tank beginners and experienced hobbyist alike. Some forms of BGA grow slow and are very difficult to kill, other species grow very quickly and can overwhelm and "smother" all the plants in short order. I have encountered some especially virulent species that wouldn't die without treating the tank with antibiotics.
Treatment Option 1:
Tank blackout. This method is very effective against most species of BGA. Manually remove as much of the slimy BGA as possible. Then do a large (50-70%) water change and cover the tank with trash bags or thick blankets for 3-4 days. No light whatsoever should enter the tank. Your fish and plants will be fine, some plants like Glossostigma may get a bit "leggy" but you can give it a "haircut" and it will return to proper form in short order. Upon removal of the trash bags/blankets do another 50-70% water change, this helps get rid of all the dead BGA. Dose 10 ppm of nitrate and maintain nitrate at 5-20 ppm throughout the week.
Option 2:
Many times if the BGA is located in just one spot in the tank you can redirect the water current to kill the BGA. Stagnate spots are prime areas for BGA to develop. Generally speaking BGA doesn't like heavy current. I have positioned a power head directed into a patch of BGA and caused it to shrink and even disappear within days. Insuring that water circulation is ample in a planted tank helps thwart BGA outbreaks. Dose 10 ppm of nitrate and maintain nitrate at 5-20 ppm throughout the week.
Option 3:
Sometimes aquarists will inadvertently allow nitrate to drop to zero and remain there for several days to even weeks on end. When plants are nitrogen starved the environment is ripe for a BGA infestation. Increasing current and adding nitrate via potassium nitrate additions can often times eliminate BGA under these circumstances, so dose 10 ppm of nitrate and maintain nitrate at 5-20 ppm throughout the week.
Option 4:
Some species of BGA are extremely virulent. Repeated changes in current, blackouts, nitrate additions...nothing is able to fully beat back the BGA. In these extreme cases it may be necessary to, as a last resort, use an antibiotic such as erythromycin to kill the bacteria. I have had success with Maracyn. Dose at the full rate of 200mg per 10 gallons of water per day and dose for the full five days. Treatment with lower doses or shorter times can lead to not fully killing the bacteria and "could" lead to a more resistant strain. Like the use of any medication this should only be considered after repeated other methods fail. Do a 50-70% water change after the full treatment and dose 10 ppm of nitrate and maintain nitrate at 5-20 ppm throughout the week.
