Low nitrate levels seem to trigger BGA outbreaks for me. Here is an article I did for Northwest Aquarist (the GSAS newsletter) after dealing with my last outbreak 18 months ago.
"Last month I mentioned that I had a “dirty little secret”. I explained that I did not bring plants to the March meeting because I have been fighting cyanobacteria, also known as Blue-Green Algae (BGA), in one of my tanks and I didn’t want to share it with others. After six weeks of trying different methods I seem to be making major headway and will share what is succeeding for me.
As I write this it has been exactly one week since I finished treating an outbreak of BGA in my 30 gallon aquarium. I believe it started because I let my nitrates (NO3) get too low and missed a couple of water changes. In addition I hadn’t cleaned my canister filter for several of months so debris had built up and the flow and circulation had dropped in the tank. Any or all of these conditions could have been the cause for my outbreak.
Cyanobacteria (BGA) is unlike most algae because it has attributes of an algae and a bacteria. They can generate food (and growth) either through photosynthesis like algae or they can convert nutrients in the water into food like bacteria. There are several different species of BGA, some of which are toxic to animals (including us!).
In researching BGA I learned that there were four methods that hobbyists commonly have tried and had success with. They are: a) a multi day complete tank blackout; b) increased dosing of nitrates like KNO3, c) antibiotics like Maracyn (Erythromycin) and d) hydrogen peroxide (H2O2).
Since this was a planted aquarium, I didn’t want to do a complete blackout for several days. I was concerned that I would lose a lot of my plants, especially the more fragile ones. I had tried several weeks of increased dosing (1/8 tsp per 10 gallons) of potassium nitrate (KNO3) which had been successful last summer when I had a small outbreak in my 10 gallon tank but this time the BGA was unaffected.
Several of the planted aquarium forums reported success using Maracyn, which has the antibiotic Erythromycin as the active ingredient, as a treatment for BGA. Apparently the “bacteria” portion of the BGA was killed by the antibiotic and it weakened the BGA so much that it would die in a few days.
Long story short, I dosed Maracyn for 8 days. The BGA growth slowed down during the treatment but it did not die. As soon as the treatment was over the BGA was growing as well as ever. I later read that there were some species of BGA that were unaffected or had built up resistance to antibiotics.
Here how I finally got rid of it. First I worked on the possible causes of my outbreak. I cleaned my canister filter and added an airstone (I would have used a Koralia Pump or powerhead if I had one) to increase circulation. I again increased my fertilizer dosage to 1/8 teaspoon (tsp) of KNO3 (potassium nitrate) and 1/64 tsp of KH2PO4 (monopotassium phosphate) per 10 gallons daily. Lastly I increased my 33% water changes to twice a week.
Next I went to work on the algae. I siphoned and manually removed as much of the BGA off of the substrate, plants, and hardscape as possible and cleaned the glass. Most of the easy-to-replace plants I threw out. I removed most of the hardscape from the tank, scrubbed it, rinsed it, and put back in place. It was very important to remove as much BGA as possible before doing the next step to minimize oxygen depletion, and fish death, from dying algae.
I treated the BGA with Hydrogen Peroxide 3.0% (stabilized) solution. Hydrogen Peroxide (H2O2) is available at most grocery stores, pharmacies, and drug stores. I also picked up a 10 ml dosing syringe (for oral dosing of medications) at the drug store when I picked up the H2O2.
I always did this part of the treatment when I knew I was going to be around for at least a couple of hours afterwards so I could watch my fish for signs of distress. I only treated the tank after the lights (about 2 WPG compact florescent) had been on for at least two hours so photosynthesis was occurring at its’ maximum in the BGA. I found that the BGA reaction to the treatment was strongest, and fastest, in the areas of the aquarium with the brightest light (and the most photosynthesis).
I turned off my filter about 5 minutes prior to treatment to minimize water movement. I dosed the 3.0% (stabilized) H2O2 at the rate of 1.5 ml per gallon of water volume (my 30 gallon tank has 24 gallons of water in it). I put the H2O2 in a glass, filled the 10 ml syringe, stuck my hand in the tank, and s-l-o-w-l-y “painted” the areas where the BGA was heaviest with the thickest “clumps”. I added the H2O2 at a rate of about 10 ml per minute so as not to stress the fish too greatly.
The effect was rapid and dramatic. After several of minutes the areas of BGA I had “painted” with the H2O2 began to bubble (gas) and started to turn a rusty brown. After about 30 minutes the “painted” areas were bubbling intensely and had turned a definite brown color while other areas of the tank (where I had not “painted”) were also bubbling (but not as intensely) and starting to turn rusty brown. After about an hour the water in the tank had a slight “rusty” color, the areas or BGA I had “painted” were a definite brown, areas of BGA that were heavy but had not been “painted” had some of the brown, and plant leaves that had a film of BGA and a film of brown. After one hour I turned the filter and airstone back on and continued to observe the fish for any signs of distress for an additional hour. After a couple of hours the water cleared and the fish become more active. After 24 hours the areas I had "shot" had turned a grey color and the BGA appeared dead.
I followed the same procedure for dosing the next day again dosing the areas with heaviest algae that was yet dead. The dosed areas bubbled and turned rusty but there was not as much bubbling and rust in other areas (I think because areas with a light coating of BGA had been killed on the first day), the water did not changed color as much, and bubbling activity and any discoloration cleared in about 2 hours.
Each day for a two week period I followed the procedure I described above. Each day the amount of BGA diminished and after about 8 days the BGA appeared to be gone. I continued the treatment for an additional four (4) days just to be sure there was no residual “hiding” in the crevasses of my driftwood or under plant leaves. It was not necessary to hit the BGA directly with the hydrogen peroxide to kill it. For example, on the edges and undersides of plant leaves I did not “paint” the areas but after about 5 days the BGA had turned rusty brown, was breaking away from the leaves, and was falling off.
As a side note I also had an outbreak of Black Brush Algae (BBA) in this same tank; probably due to the low CO2. The treatment described above worked on it as well, but the BBA did have to be “painted” to die and areas in bright light responded the best to treatment.