Fighting Blue-Green Algae with Erythromycin

by Christopher Petrides

The first time I saw a planted tank I was truly amazed by the beauty and colors of the plants and layout. I wanted to create something similar, and as a beginner, the first thing I managed to grow in my planted tank was algae, of course! Not only one type, but several. The latest type was the one called Blue-green Algae.

There are many articles in e-flora and web sites about Blue green algae. I will attempt to give some basic info that I read about it (I am not a chemist), as well as the treatment I used in order to get rid of it.

First of all, Blue-Green Algae are not algae. They are a kind of bacteria, called cyanobacteria. They are known to be quite hardy and able to survive in cold as well as really hot water. Since they are not the normal type of algae, forget the normal methods of removing it, such as keeping the lights off, or removing by hand and making partial water changes, etc.

A few words about the tank that I treated: It is a normal 60 cm x 30cm of about 55 - 60 lt depending on the gravel, rocks, and stuff you put in it.  The tank was heavily planted with "easy plants" that were growing like crazy. Only an internal pump and an undergravel filter drove it. The tank was running for at least 12 months with no problems when BGA started to grow. Since many Cyanobacteria do not depend much on nitrite, nitrate, or ammonia (which were almost unmeasurable in the tank), it had to be something else. I did not take any other measurements at that time, but I know that dirt was starting to pile up in the under gravel filter, since I could not vacuum the entire area (lots of plants you see...).

BGA started growing amazingly fast. No matter the water changes, it was covering almost any surface wide enough that existed, including the gravel, plant leaves, rocks, wood, everything. It was a dark green color, and one could remove an entire piece by hand.

Since I knew that it was actually bacteria, the first thing I tried was to expose them to the atmosphere, in order to kill them. So I removed the fish, and lowered the water in order to expose the gravel. I knew that I was probably going to terminate all my good bacteria also, but at that time the only fishes I had were my young fighting fish (Betta splendens), so compared to the amount of plants, the bioload was really small. An extra vacuum of the gravel also took place and I also removed some plants to free up space.

Well, I wasted my time, because BGA was back, and really furious. In fact, in a matter of days, I had a really nice Amano style coverage on my plants and gravel.

It was at that time that I read about Erythromycin. It is not supposed to hurt fish and plants, and from what I read most of the biological filter remains unharmed. Since I prefer to use human medicine and antibiotics instead of the ready-made medication available in the hobby (a trend which is actually owed to George Reclos and his precious advice), I rushed into a pharmacy asking for the equivalent antibiotic that was based on Erythromycin. It was called Erythrocin, and surprisingly enough, you don’t need a prescription to buy it. It comes in two forms, 500 mg capsules and spring tablets (the ones that melt in water). A box of 12 capsules only costs at about € 3,50. Imagine how many treatments you can do, especially if you keep small tanks :).

The dosage that is most commonly used is no less than 2,5 mg per litre. So for my 60 lt tank (actually less if I consider gravel, plants etc.) I needed 60 x 2,5 = 150 mg. Since I like to overdose a little bit (another trend by George Reclos :) ), I split the 500 mg capsule in 3 parts (166 mg) and decided that this would be one dosage (treatment).

First day

I dissolved a 1/3 of the capsule completely in a small basket containing tank water. When you spill it in the tank, you will notice small white dots swimming around. It is the medicine. Actually, if you can find the spring tablets, it is much better.

I watched (in agony, I must confess) how the fish would react. They did not seem bothered at all (Some of them were actually hunting the white dots as they thought it was food...Silly bettas... when they are young ;) ). No sign of stress or discomfort.

It is important not to have anything that might decrease the effectiveness of the medicine, like a protein skimmer at that stage.

Second Day

Added 1/3 of the capsule. You should notice extra bubbles on the BGA bed, and you might also see pieces of BGA trying to leave the gravel and head to the surface.

Third day

By the end of third day you might see large pieces of BGA slime floating around. Mine was quite full. Record your ammonia and nitrite levels, and perform a 30% water change if they are high. Try to get rid of the slime by other removing it with a net (as I did) or by using an extra mechanical filter.

Fourth day

By the end of the day most of the plants were clean. There were still some bits and pieces around, but the tank looked much better. The fish and plants were unharmed, like nothing was going on, and everything seemed normal. I performed a 50% water change and added 1/3 of the capsule. I again measured ammonia and nitrite levels.

Fifth day

My tank was almost clean now. The important thing after the third day is the measurement of ammonia and nitrite.

Sixth day.

What happened to me:

BGA was completely gone. The results were amazing. I stopped treatment, performed a large 70% water change, and then resumed normal maintenance.

Other treatments suggest:

It has been reported that in some cases erythromycin may also harm the useful nitrifying bacteria in the tank. If this happens in your tank (which will be evident by the increased ammonia and nitrite levels in your tank) then you can perform a smaller water change (like 30%) and add half the recommended dosage erythromycin. Then resume the normal maintenance of your tank.

Normally you should not have any problem with your fish and plants. Although I have the worst filtration in my tank (under gravel filter!), bettas are quite hardy compared to other fish. The tank was also small, so I could easily perform large water changes if something seemed wrong. Since my plants were also in good growth, I felt confident that I would not have an ammonia break during treatment. That is the reason that I did not take any measurements, and of course, the fact that I had run out of tests! If you have delicate fish, then you need to measure daily in order to be on the safe side.

Since then, BGA has appeared once more after a period of two months. I did the same treatment; only this time the cycle was shorter. In fact, I had only reached the third day and it had disappeared. Since then, BGA has not appeared again. I assume that overdosing worked, killing all cyanobacteria in the tank. If it doesn't, then it is most likely that you will end up with a really strong bunch of cyanobacteria! Many people say that although they treated their tanks BGA returned after some time. Actually, I thought that this was happening to me as well a few days ago, but after a closer look (and lots of pruning in the plants) I discovered it was normal green algae. Well, one cannot have it all. Better to have algae than blue-green algae!

I had left bits and pieces of Riccia floating, in order to grow and then attach them on wood. Those were on the surface but on the back side of the tank, where I could not see what was happening... When I decided it was time to attach it to wood, I grabbed it and... oh what a surprise, a very thick layer pf BGA had been formed on top of Riccia. In fact I had noticed really small bits on the gravel as well, but since it was not a lot I did not worry... I cleaned the Riccia as much as I could (a really tough task !) but there were still bits that remained attached. However I did lay it upon wood and placed it in the tank. On the next day the tiny bits had transformed to the thick green blanket that you see in the picture ! I started another treatment cycle just to see how strong this is, since I had treated the tank two weeks ago, and I assume that the cyanobacteria grew stronger.... I will keep you informed with another update.

Riccia fluitans covered with a thick layer of BGA

Some additional notes about Erythromycin: Actually named Erythromycin A [ C37H67NO13 ] it is an antibiotic substance produced by a strain of Streptomyces erythreus (hence the name "Erythromycin") found in a soil sample from the Philippine archipelago. There are three erythromycins produced during fermentation, designated Erythromycin A, B, and C; A is the major and most important component. Solubility in water : 2 mg/ml or 2 g/L. Therapeutic category : Antibacterial. Erythromycin is used as a salt (most commonly succinate salt) but the strength (e.g. 500 mg) always refers to the basis (erythromycin).

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