10-31-2010, 06:39 PM
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The following may help you [it is not mine, it was written by Dusko Bojic for another site]. As always, be very careful if you decide on using antibiotics. I have never resorted to this; some plants will be harmed and even killed with erythromycin (pygmy chain swords come to mind). The cause is organic--regular large water changes help. RTeducing light even to a blackout is sometimes suggested, but this can also harm the plants. And any method that does kill cyanobacteria will result in a lack of oxygen (due to the decomposing mass) and high ammonia/nitrate.
even though it's commonly called blue-green algae (BGA), it's not classified anymore as one. This "algae" is actually cyanobacteria, a form of life that has both animal and plant characteristics. It forms slimy, blue-green, sheets that will cover everything in a short time and give off a strong, characteristic scent. If left to over-run the tank, cyanobacteria may kill plants and even fish. It doesn't stick and can be easily removed manually, but will return quickly if the underlying water quality issue is not fixed. It can be treated with Erythromycin and other antibiotics, but this method should be done carefully since it might affect the nitrifying bacteria in the gravel and filter, and improper use of antibiotics always brings the risk of developing a more resistant strain. When the BGA gets killed by the algaecide it will start to rot and through that process it will reduce Oxygen levels in the tank. Since the nitrifying bacteria needs O2 to transfer ammonia/nitrites into nitrates the nitrifying process will slow down. If algaecide is used, make sure to test the ammonia/nitrite levels. Remove all the visible algae to prevent it from rotting in side the tank.
Some aquarists use the black-out method previously described, where black bags are wrapped around the tank for 4 days and held in complete darkness. It is advisable to raise NO3 levels to 10-20 ppm before starting the black-out period. Manually remove as much BGA as you can before the blackout, and dead matter after the blackout.Egeria densa (Elodea) and Ceratophyllum demersum are good plants to have in a tank, since these plants are known to secrete natural antibiotic substances that can help prevent BGA. Establishing lots of healthy, fast-growing plants from the day you start the tank, dosing the nitrate levels to maintain 10-20 ppm, and vacuuming the gravel to keep the tank free of decaying matter is the best way to prevent this "algae". BGA can be found in aquariums with very low nitrates because it can fix atmospheric nitrogen. BGA seems not to like high CO2 levels and stronger water currents.