Bluegreen algae is not really an algae, it's actually one of various cyanobacteria which forms bluegreen slime-film covering everything within your aquarium in no time.
The slimy-film is made by cyanobacteria as a net to catch food and as protection against any threat, including toxins & antibiotics. The prospective bacteria may come from any source : your tap water, newly-bought fish or aquarium plants, etc. You can't see the miscroscopic bacteria, but they are probably there -floating around or buried under the aquarium substrate- waiting for suitable enviroment to develop, multiply and form a group then make a slimy covering.
After the slimy-film formed, this blue algae is very difficult to be removed, especially for more established one. If it already severely-infected your aquarium plants, you'd better throw your beloved plants away.
This blue algae is also stink when it is removed from the water, you'll be frustrated if you try to remove the slimy-film manually -especially when cleaning the infected aquarium plants. You are lucky if they haven't infected your aquarium plants yet, but quick actions are needed to stop the spreading.
How to effectively combat the bluegreen algae has been a debatable issue among aquarists for more than a decade.
There are many references around, some are good, some aren't. In my opinion, in combating the Bluegreen algae, we should know its specific characteristics and start the treatment from there. You may consider the followings :
Bluegreen algae is an infection. How to treat an infection? Usually by using an antibiotic. The problem is How to find an antibiotic that is safe for aquarium plants, fish or other inhabitants including beneficial bacteria living in aquarium substrate and biological filter media. Erythromycin antibiotic is highly recommended by many aquarists for over a decade. Erythromycin is one of the safest antibiotics -meaning that it does not affect inhabitants (plants, fishes or beneficial bacteria)- based on several facts :
It is already a fairly useless drug for fish diseases.
The bluegreen algae is more sensitive to it.
Fortunately, the beneficial bacteria important for the nitrogen cycle are much less sensitive to Erythromycin than the cyanobacteria (the bluegreen algae).
The cyanobacteria are normally reduced to a less-than-visible population within less time than the treatment requires. Perhaps not wiped out (becoming dormant), but certainly reduced to a point where it doesn't rebound in the short term. Long term preventions should be made soon after your tank are 'freed' from this blue algae.
Some suggestions for battling bluegreen algae :
From "Blue-green algae and erythromycin" article by Tony Clementz at Tony.Clementz.mikrbiol.lu.se :
First make sure that it is blue-green bacteria and not the “normal” algae. Remember that erythromycin is ineffective on anything but bacteria. Day 1. Add 2.5 mg/L erythromycin. If you have a protein skimmer, turn it OFF. I believe it will inactive a lot of the antibiotic through coprecipitation with protein. It will, however, be very usefull later. Day 2. Add 2.5 mg/L erythromycin. Day 3. Now you should see a lot of dead blue-green bacteria floating around in the tank. Increase filtration (a second mechanical filter if possible) to get rid of it. If you have a protein skimmer, turn it ON. The critical thing now is to get rid of as much protein (dead bacteria) as possible to avoid a peak of ammonia. Day 4. Most blue-green bacteria should be dead by now. Try to clean out as much as possible of the dead stuff. I use a jet stream of water from the outlet of a canister filter to remove it from plants and decorations. Combined with the second mechanical filter, this works fine for me. Let the filter work for a couple of hours then make a 50% water change. Add 2.5 mg/L erythromycin. Day 5-7. Wash the mechanical filter at least once a day. Keep check on ammonia and nitrite but do not change any water unless absolutely necessary. The extra filter can be removed as soon as the water clears up. Day 8. Make a 30% water change. Add 1 mg/L Erythromycin. From now on, resume your normal maintenance.
Another experience (from "Fighting BlueGreen Algae with Erythromycin" article by Christopher Petrides at malawicichildhomepage.com) told that human antibiotic that was based on Erythromycin s.a Erythrocin can be used using the same dosage (2.5 mg/L) and above-sequence for treating the bluegreen algae. The result is surprisingly effective, the bluegreen algae was completely gone in the 6th day of treatment.
Algaecides are toxic chemicals that are widely used to prevent the growth of nuisance algae, if used indiscriminately, it can kill inhabitants. Dichloron aphthoquinone is selectively toxic to bluegreen algae. Don’t choose treatment which contains copper inside, copper is poisonous to everything – aquarium plants, fishes, and bacteria. Alhtough all algaecide products claim that they won't harm other aquarium life, not even plants, some aquarium plants (s.a Vallisneria gigantea or Echinodorus family) are very sensitive to chemical treatment - indicated by melting leaves (turn into leaves-porridge), faded leaves, brownish spots on entire leaves, etc.
Cyanobacteria are photosynthetic bacteria, it have an elaborate and highly organized system of internal membranes which function in photosynthesis.
Black-out method for 4-6 days -as usually adviced by some aquarists- only stops their photosynthesis process, NOT to kill them. The blue algae becomes dormant and will active again more fiercly when you turn the lights back on.
Cyanobacteria are able to fix nitrogen gas into ammonia (NH3), nitrites (NO2) or nitrates (NO3). Their dominance becomes favored at LOW nitrogen levels, not high. Some references said that the bluegreen algae arefavored when the Nitrogen:Phosphate ratio becomes low (less than 16:1). On the contrary, the growths of diatoms and other green algae are favored at high Nitrogen:Phosphate levels. This low Nitrogen level usually happens in planted tank with very low inhabitants : dense of aquarium plants with one or a few little fishes. That's why, the bluegreen algae doesn't appear in fish-only tank which full of nitrates (NO3) inside.
Bluegreen algae or cyanobacteria don't like acidic environment. Generally, pH value between 6.5-7.5 is suitable for almost all fishes and aquarium plants. Amazon fishes such as tetras (cardinal, neon, black-neon, bleeding heart, red-nosed etc.), cichilds (discus, angelfish, etc.) prefer slightly acidic water (pH around 6-6.5). Especially for Cardinal tetras, they can tolerate pH level between 5.0-7.0 and give their best-coloration at pH 5.8.
If your aquarium ever infected by the bluegreen algae, you should create an environment that is unfavorable for them, there are possibilities of resistance or remain-dormant cyanobacteria after the treatment!. Use an aquarium CO2 system to set the pH value at a slightly-acidic level as a preventive measure.
Some references claimed that the growth rate of the bluegreen algae declined when the aquarium CO2 system operated. The bluegreen algae didn't disappear suddenly but it did cease to grow and start disappearing. Water turbulence usually prevents Bluegreen algae reaching the surface, NOT to limit their growth or eliminate them. If, however, turbulence suddenly weakens, the buoyant population may float at the water surface.
Repeated cases. If you already killed the bluegreen algae with erythromycin antibiotic, were freed from the problem for awhile, then be haunted again by the same problem, you can suspect the followings :
1. There are dormant cyanobacteria hiding in your tank which means that the aquarium treatment is not completely success. Some favourable conditions will activate them such as :
pH level is not maintained at weak-acid level, pH<7 is unfavorable environment for this blue algae. It's difficult if you kept the fish that need more alkaline water (pH >7) s.a Lake Malawi (African) chichilds or live-bearer fishes (platty, guppy, molly). So, first you should repeat the aquarium treatment -this time do it as thoroughly as possible, then eliminate the prospective bacteria before it entering your aquarium (see point #2).
Nitrate (NO3) level are too low, you can generally expect an outbreak of the bluegreen algae.
2.Your tap water contains prospective cyanobacteria. You should use UV filter to kill all micro-organisms before the water entering your tank.
Conclusion :
Bluegreen algae or cyanobacteria are very difficult to be completely eliminated. Once your tank is infected, it most probably will have a relapse in the near future. To overcome this condition, short-term treatment should be combined with long-term preventions.
Short-term treatment :
Use Erythromycin antibiotic or other antibiotics such as Maracyn (which has Erythromycin as active ingredient) rather than algaecides. Nowadays, it may be difficult to find fish medications with erythromycin inside. If it's the case, you can use human antibitotics s.a Erythrocin as replacement. Dissolve it with the tank-water and dose your tank as above explanation.
Long-term preventions :
Check the pH level, use an aquarium CO2 system to maintain the pH level at slightly-acidic level (around 6-6.5), it depends on the the fishes' origin.
Check the Nitrate (NO3) and Phosphate (PO4) levels. You should be very cautious here! There are 3 limitations that should be balanced : safe Nitrate (NO3) level (20-40mg/L), safe Phosphate (PO4) level (< 1 mg/L) and the N:P ratio (16:1 at a minimum). For example : if your water parameters are 20 mg/L of Nitrate, 0.8 mg/L of Phosphate with N:P ratio at 25:1, you are preventing the bluegreen algae to relapse and also the 'real algae' to bloom.
If a slightly-adicid water doesn't enable for your beloved fishes, you can use this devices :
Ozonizer. It can kill all microbes in your tank effectively (including all beneficial bacteria). It's not the safest choice, but the risk can be reduced by automatically controlling the ozone level using a special device to reach the safe and effective aquarium ozonation. After that you can return the healthy condition back by adding bacteria preparation to speed-up the increasing number of beneficial bacteria.
UV filtercan ONLY killl free floating micro organism using high intensity ultraviolet light, NOT the cyanobacteria that are hiding somewhere in your tank. It's not effective to permanently eliminate the bluegreen algae, but it's very useful for purifying your tap-water as a preventive measure.
Egeria densa (Elodea) and Ceratophyllum demersum are useful aquarium plants to have in your tank, they secrete antibiotic substances which can help prevent the blue algae.
Finally, I hope this article can answer your confusions on How to effectively treat the blue algae. See you in our next inspiring articles.
Aquarium Algae 
is highly recommended by many aquarists for over a decade. Erythromycin is one of the safest antibiotics -meaning that it does not affect inhabitants (plants, fishes or beneficial bacteria)- based on several facts :
