Cyanobacteria are a gram-negative bacteria which is often classed under algae by fish keepers because of its ability to photosynthesise. It spreads across all surfaces, coating plants in a layer of slimy-looking goo. In fact, the goo may actually feel quite solid, rather than slimy. There are many different species of cyanobacteria, including the Spirulina genus, which is used in the manufacture of some vegetable based fish foods. The cyanobacteria commonly seen in the aquarium are not as useful, unfortunately, often being from a range of species which specialise in nitrogen fixation. Nitrogen fixation is the conversion of N2 into NH3, this may potentially cause problems. Some species of cyanobacteria are also capable of producing cyanotoxins, which may harm fish.
The biggest dangers cyanobacteria poses in aquariums is killing the plants by physically cutting off light and using up oxygen.
Since the plants can reduce the chanced of the bacteria reappearing in the future, it is important to make sure that the plants are not killed off by physically removing as much of the cyanobacteria as possible. The next step is the treatment. There are two options:
- Blackout: clean off as much bacteria as possible, then cut out all light for at least a week. This method focuses on the bacteria being starved of light and nutrients (as no fish food would be going into the water). It may take a few attempts to truly kill off the bacteria and each attempt should also be followed by removal of any remaining bacteria. A week without food will not harm most tropical fish, so this method is relatively safe and sound.
- Antibiotics: an antibiotic which acts against gram-negative bacteria may kill the bacteria. This option needs to be considered with care because most medications are harmful to aquatic animals (the extent to which this is the case depends on many factors, including the pH of the water). Some antibiotics which are sold as treatments for fish will also harm the gram-negative nitrifying bacteria.
Given that there are no fish in the aquarium, I decided to try the antibiotics method. A quick search on the internet showed me that eSHa 2000, a generic antibacterial and anti-fungal medication which is produced by eSHa Labs, may work against cyanobactria. A further search showed that the ingredients are ethacridine lactate (antiseptic, trade name Rivanol), proflavin (antibacterial against gram-positive bacteria and antiseptic), Cu2+ (antimicrobial) and methyl orange (mutagen). Since I was unable to find whether the ingredients acted against gram-negative bacteria, I emailed eSHa to ask. This is the reply I received:
eSHa 2000 does work against [gram-negative and gram-positive] bacteria but not against all bacteria.
I proceeded with a standard treatment, following the instructions on the medication. On the first day, I added 14 drops, followed by 7 drops on each of the following days. By the second day, most of the bacteria was gone. It has now been one week and there are no signs of cyanobacteria left, so I can conclude that the medication or some other combination of factors have resulted in successful treatment.