Algae – Blue Green (BGA) & Green Spot

February 26th, 2008

At the last GWAPA meeting, I gave a presentation called Algae in the Planted Aquarium. While preparing for the presentation, I had to gather a lot of information from a number of different sources on the Internet. I’ve decided to declare this week Algae Week, and share that gathered information by posting about two types of algae each day. This is the second installment featuring Blue-Green (BGA) and Green Spot Algae.

Blue Green (Cyanobacteria)

Blue Green Algae (BGA)

While often referred to by aquarists as an algae, Blue Green Algae (BGA) is in fact a bacterial slime that can easily coat everything in your tank. Appearing as either a green, black, or purple coating, BGA is perhaps best known for the unique earthy smell that it has when pulled from the tank. As a nitrogen-fixing bacteria, it will fully deplete your water column of any available nitrogen.


  • Low nitrates – Usually present when all of the nitrogen/nitrate has been removed from the water column. While this is a triggering condition, it is also exacerbated by the bacteria itself using any remaining nitrogen.
  • High organics – Overfeeding, or excess organic matter in the tank can trigger BGA.
  • Old light bulbs – Sometimes present when light bulbs are no longer emitting usable light. This may be more of a matter of your plants no longer being able to out-compete the bacteria.
  • Poor water circulation – Circulation is key in a planted aquarium so that no “dead spots” are present where nutrients have been used up locally, but fresh ones are not being recirculated throughout.


  • Increase nitrates – Dose nitrates until the concentration reaches ~5ppm.
  • Add fast growing plants – this helps to out-compete the algae for resources.
  • Blackout – BGA cannot survive without light.
  • Excel/H202 treatment – Use a syringe to spot treat problem areas. Then manually remove dead patches.
  • Erythromycin – use antibiotics at half dosage to kill the bacteria. Mardel Labs’ Maracyn contains erythromycin and has been used effectively without harming most plants.

Green Spot (Choleochaete orbicularis)

Green Spot Algae

Green spot algae is very commonly seen on the glass of tanks when there hasn’t been a water change in awhile, or when an inadequate fertilization scheme has been conducted. GSA also appears on long lasting leaves, such as Java Fern, Anubias, and Bolbitus.


  • Low phosphate (PO4) levels – almost exclusively caused when phosphate levels are depleted.


  • Scrap glass – Use a razor blade to most easily remove from the glass.
  • Dose Phosphates – Dose PO4 to a concentration of 0.5-2.0ppm.
  • Nerite Snails – Nerite snails can help you remove green spot from leaves, as well as, the glass.


Aquatic Plant Central – Algae Finder

12 Responses to “Algae – Blue Green (BGA) & Green Spot”

  1. Tennessee Mom Says:

    I have both of these! The Cyanobacteria is interesting because I think it’s related to the light. I have a corner 55 which is 24″ deep and even though the plants grow in there, they are covered in the stuff. I don’t have proper lighting (using 1 65watt 12k saltwater bulb) But, if I take a plant out and put it in my 10 gallon shrimp tank with 48w of proper freshwater lights, between the shrimp and the lighting it’s gone the next day.

  2. guitarfish Says:

    BGA, in my experience, is very much linked to the light, but maybe more so the nutrients and circulation. I’ve found a number of “deadzones” in my tanks, where all I needed to do was add a small powerhead to push water through the area, and the problem went away. Similarly, when I don’t dose enough macro-nutrients (NPK), particularly nitrogen, it creeps up. Of course, if your bulbs are old (> 1 year), it’s also possible that could be the problem, but I would look to the afore mentioned possible causes first. In a tank that’s admittedly low-light like that, sometimes all you need to do is dose some Seachem Flourish/Excel every couple of days to keep the nutrients from bottoming out. Question: Do you notice the BGA in any specific areas? Top/bottom, one side or the other, etc…

  3. Tennessee Mom Says:

    I do dose Flourish and Excel every few days. Current is interesting, and I was thinking of getting something that could add more current.

    When I adjust the spraybar downward, the fish are all at the top and my pleco comes to the top. But when I have the spraybar rippling the surface like I normally do, the fish are all fine.

    I think I need to get a powerhead. they are cheap enough that I think I could get one without too much hassel from hubby:)

  4. Tennessee Mom Says:

    I took a picture this morning. Now that I see it close up, maybe it’s that beard algea you talked about. Here’s the link to the picture

  5. Tennessee Mom Says:

    Oh one more thing.. my husband all last week kept complaining that the tank made the room smell like “dirt”. I haven’t noticed any offensive smell, but then I’m out outdoorsey type. I put some charcol in the filter, and he said it went away. Could this be the algea odor you were talking about?

  6. guitarfish Says:

    Very well could be, Tn Mom. If you pull a plant out that’s got BGA on it, ask your husband to smell it, and see if it’s the same smell. OK, that might take it too far! 😉

  7. guitarfish Says:

    Regarding the picture you sent. That could be diatom algae, or “brown” algae. I have a post coming on that. Stay tuned!

  8. Tennessee Mom Says:

    My mind has gone downhill since I turned 40. All last week I kept spelling angelfish “anglefish”. Now I see that I keep spelling algae “algea”. 🙁

    Can you tell me what plant this is? . Yesterday it had a little stalk, by tonight it’s almost to the top of the tank, that’s 24 inches! It’s one of those bulbs you get at Walmart. I forgot to read what it was on the package. 🙁 Maybe there will be a flower!

    I look forward to tomorrow’s ALGAE lesson!

  9. guitarfish Says:

    Tn Mom, my best guess is that it’s a flower stalk from whatever plant it’s shooting out of. Could be a sword plant, aponogeton, or similar.

  10. Green spotted mold on rock - MyFishTank.Net Freshwater Saltwater Aquarium Fish Forum Says:

    […] is at the bottom. I bet you have green spot algae, though. This page has great pics of both: Algae – Blue Green (BGA) & Green Spot– Guitarfish Green spot algae is indeed an algae, but it adheres very tightly to surfaces. No fish I know of […]

  11. FloridaMom Says:

    Help. I have green spot all over my aquarium glass for the first time in 23 years. It is a 65 gallon built-in vertical tank, that had the old under gravel filters. The new filter has hoses over the top. I’ve stripped it down once and gotten rid of the green spot, but it’s back. Could it be the new filter or sand at the bottom (I used to have stones)? Thanks.

  12. guitarfish Says:

    FloridaMom, chances are that your old under-gravel filters contained a lot of mulm that leached small amounts of nitrate/phosphate into your aquarium, which your plants could use for nutrition. The new substrate and filter are brand new, thus very clean, and are not providing any nutrients for your plants. When Green Spot algae occurs, it is almost always because your phosphates are depleted (gone). Without knowing your exact tank conditions, it is difficult to prescribe an exact regimen for you, but my advice would be to scrape the GSA from your glass, and start dosing small amounts of phosphates to see if the problem goes away. In low-light tanks, this may mean just regularly dosing a comprehensive fertilizer mix like Seachem Flourish several times a week. In high-light/CO2 tanks, you will likely need to dose KH2PO4 separately. Good luck!

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