Blue Green Algae Article


Blue-green algae and erythromycin
by (Tony Clementz)
Date: 7 Feb 92
Newsgroup: rec.aquaria

I've followed the postings about blue-green "algae" and Maracyn (erythromycin) the last few weeks and the following is an attempt to review some facts about erythromycin (the active ingredient in Maracyn) and blue-green algae, with special emphasise on the use of erythromycin for treating blue-green "algae" infections.


Thats right. The correct name is blue-green bacteria (or cyanobacteria for those latin freaks out there). So without going into a lot of details, blue-green algae is a bacteria so an outbreak of the stuff in your tank is actually an infection.

It is important to know that blue-green bacteria comprise a large and heterogeneus group of organisms. Not even the color is the same. Some are green, some blue-green, and some are red. They can be found almost everywhere in nature. They are usually more tolerant to extreme environments than "normal" algae and can be found in hot springs as well as saline lakes. Drying your gravel and tank is subsequently not an efficient way to get rid of them. Some species can even be found in the middle of the dessert. Blue-green bacteria efficiently absorbes light between 550-700 nm, which is roughly the same as for plants and green algae.

As we all know, they thrive in warm water, rich in nutrients. However, many blue-green bacteria is not dependent on nitrite, nitrate, or ammonia, since they can use molecular nitrogen (nitrogen fixation). This all leads to the well known conclusion - once established in the tank, they are a pain in %&#-%&. I wonder how many potential aquarium hobbyist has been lost because their first tank became covered in green slime within six month.


Bacteria can be divided into two groups, either Gram- negative (G-) or Gram-positive (G+). This classification is based on if the bacteria stains (+) or not (-) in a special staining technique - the Gram staining (invented by Christian Gram). Positive or negative staining reaction reflects a fundamental difference in the structure of the cell wall of the bacteria.


Erythromycin is more efficient towards G(+) bacteria than G(-). It is one of the safest antibiotics, meaning that it does not affect plants, fish or animals. Blue-green bacteria belongs to the G(-) bacteria but it is a special case with respect to sensitivity to antibiotics (i'm on thin ice here, but I think I am correct). They are more sensitive to erythromycin than other G(-) bacteria. Fortunately, the bacteria important for the nitrogen cycle (your biofilter) are of the G(-) type and are much less sensitive to erythromycin than the blue-green bacteria. So your biological filter is "fairly" safe.

The reason that some tanks experience an ammonia peak after treatment with erythromycin is (probably) not because the biological filter is non-functional. It is more likely that it is because of the high content of protein released from the dead blue-green bacteria which is broken down to ammonia and/or nitrite by the "good" nitrifying bacteria in your biofilter. This boost of protein to be broken down upsets the finely tuned balance of different bacteria in your filter. (Actually, if you killed of all bacteria in your tank and filter, you would never get ammonia).

In many countries in Europe there are restrictions on buying antibiotics. You usually need a prescription. I suggest contacting a vet. If he can prescribe antibiotics for a mouse I'm sure he can do the same for your tank. Remember, your tank is infected.


First a few things NOT to do (my own, very personal, experience).

If you have an established infection, do not try to get rid of it by turning the lights of. Most likely this will get you into more trouble. Your going to kill of the "good" algae and the plants, but the blue-green bacteria is going to return when you turn the lights back on (usually more fiercly than before).

Personally I don't like copper. Copper is poisonous to everything - plants, fish, and bacteria. At least in Europe, most "miracle" treatments you buy contains copper.

You could try manually removing the blue-green algae, combined with extensive water changes. But in my experience it's fruitless, unless you spend all your free time with your fingers in the green slime. Ever tried to clean the stuff away from Cabomba or Java fern? Then you know what I mean.


Of course, tank hygien is important. Regular water changes and all that. But for those who has been doing it all according to the textbook and still wondering if your doing something wrong, don't despair, you have'nt been hit by a blue-green curse.

Me myself, I get an infestation in about every second tank (freshwater) I set up. These does not correlate to any increase in ammonia, nitrite, or nitrate (I do not keep check on phosphate, but I plan to). One exception is tanks with soft and acid water. You rarely find blue-green bacteria in these tanks since most bacteria, including blue-green, do not like acid conditions.

The followin is my suggestion for battleing blue-green algae:

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.

The concentration I use is in theory a bit high and getting up where it should starts having an effect also on G(- ) bacteria. When I started using erythromycin I had problems using lower concentrations in the tank. It was not very effective. It might be time to check it again.

One word of caution. Only use erythromycin when you really need it or you might end up with blue-green bacteria resistent to the antibiotic.

My practical experience of using erythromycin to battle blue-green bacteria is limited to my own few tank (and some friends). It would be interesting to get some feedback from people with experience (good or bad) of battling blue- green bacteria (with or whithout erythromycin). I would of course post a summary of the response.

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{ 16 comments… read them below or add one }

karen April 14, 2007 at 9:18 am

i need help with a questions about my tank.can you help me?

Pat Harris March 7, 2009 at 4:03 pm

We just survived the worst ice storm in over 50 years here in Brookland, Arkansas the last of January 09… I have over 370 gallons of freshwater tanks and they all went through being without electricity for over 4 days pretty good except for my 100 gallon tank… It is loaded with large Rainbows, clown loaches and many other smaller fish… A large Rainbow had died during the power outage and got caught under the air curtain where I could not see it in the black out tank, I then lost about 10 more large Rainbows and dipped out quickly at they came to the top… Of course, the quality of the water suffered greatly with only a battery operated air pump, even though I was changing water out of the tank during the outage… Shortly after we got our electricity back I started seeing the “Blue-Green Algae” on some of the Cabomba and other tall plants and now is spreading throughout the tank, the tank is heavily planted…
My question is, just how many of the packets of Maracyn do I use in the 100 gallon tank??? You say 2.5, that’s more than the one pack per 10 gallons, is it not??? And if I’m reading right, you only say to put the Maracyn in on day 1,2,4, and double dose on day 8, is this correct????
Thank you for a quick response so I can get started and I will need to order more Maracyn if this is the case….


grayson September 8, 2009 at 12:05 pm

“Some species can even be found in the middle of the dessert.”
Gross!! I would fire my wife if she ever served me such a dessert! ; )

Thanks for the info.

Robert August 16, 2010 at 3:59 am

I had more than 4 jears problems with this terrible blue green algae. I tried the dark cure twice, doesent work, I maked my 400l tank new. get all out, desinfected and make new, doesent work.
Now I have done this tip from Tony. And indeed in 8 days is my tank clean and the plants grows very good.
Many thanks for this Tony

Kind regards Robert

Fabian February 3, 2011 at 7:36 pm

I have problems with Blue Green Algae, too!
Im living in Germany. So it isnt easy for me, even as Paramedic to get Antibiotics. Thanks God I had a bad Tonsillitis an I have still 3 different Antibiotics at home: Clindamycin, Penicillin, Clarithromycin.
Which one I should use and which dose would you recomand?
Thanks for helping and best wishes

Maggie April 20, 2011 at 10:14 am

Too bad I’m allergic to erythromycin. Death of cyanobacteria = death of fishkeeper. I keep up a low-grade war at all times with cleaning and it stays under control. PS. Do I remember that Amano shrimp eat blue-green algae?

nathen June 22, 2011 at 7:38 am

i just visit your blog and gain some thing about blue green alge.
please update your blog for i know about more information

Chris September 25, 2011 at 12:33 am

Thank you so much for taking the time to write this up! I owe you so much, I’ve battled with this bacteria for years and have finally ended the struggle thanks to you. My flora and fauna are also greatly thankful.

minakshi October 31, 2011 at 4:34 am

i will like to ask about my problem of my tank
this info gave me lots of things which i needed
tthhaannkkss!!!!!!!!!!!! My fish tank . net

Hitendra April 11, 2012 at 10:45 am

I king with Oscillatria tenuis.While subculturing i have observed that my culture has got bacterial contamination.Can you suggest me some antibiotic safer against algae but effective on bacteria.according to me broth culture is contaminated with Gram + and Gram – bacteria.
Kindly suggest be safest dose to eradicate these contaminant.


Harekrishna October 7, 2012 at 8:08 pm

Let me clear….why blue green algae is called as cynobacteria.i think cyno means blue green but…why it is bacteria

Mike July 20, 2013 at 7:53 pm

I cleared this stuff, which grew in sheets on top of everything, with one two-day treatment of e-mycin (2.5g/l) a few years ago.

I have another variety now since I moved which resisted this treatment, so now I am trying Ron’s regimen.

Blech!! The new stuff is very fibrous and very tough. My algae eaters can hardly get past the stuff to the tasty green stuff underneath, and it can get tangled up in gills.

kenny jensen - Sweden September 6, 2013 at 4:34 am

Hi,, it works fine i my saltwater tank, I have tried all possible methods but none works really well, but this method works really well. The hardest thing is to get hold of Antibiotic in sweden.
Thank you for your help.


Annie February 3, 2014 at 8:15 pm

I’ve been battling BGA for nearly 6 months now, and it’s worse than ever. I did everything short of Erythromycin and I’m planning on doing it this week after reading your very informative, detailed post. Thank you so so so so so much for taking the time to write this and make it available. I had come across a few sites that told me I’d never be rid of it and I was getting very discouraged.

One thing, do you recommend that the biological filter be taken out and stored in tank water in the refrigerator? I read an article that mentioned that’d be the only way to avoid “de-cycling” the tank. I am hesitant to do that because it would mean that there would be residual cyanobacteria living in it. It would ease my mind to get your thoughts on that. Still, I do want to thank you again (so much) for giving me hope again!

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