Cyanobacteria: Causes and Remedies

What are cyanobacteria?

They are single-celled organisms, also known as blue-green slime algae because, like many algae, they tend to spread as a sort of blob, in colours ranging from green to red or blackish. In ideal conditions they can quickly cover anything in the aquarium: rocks, glass and, in the worst cases, even sessile invertebrates (stony and soft corals, sponges, etc.).

In reality they are not algae, but opportunistic photosynthetic bacteria (cyanobacteria) that thrive wherever there are ideal conditions of light (even little light) and nutrients. Their photosynthetic activity shows in the many little bubbles trapped in the slimy film.

1. 1On the rocks the film grows as a blob
2. 2On the glass and on everything in the tank
3. 3On invertebrates stony and soft corals, sponges: in the worst cases
4. 4Bubbles trapped: a sign of photosynthesis
5. 5Green, red, blackish the colours it can take

They look like algae, but they are opportunistic photosynthetic bacteria.

Why they appear

They are more common in the marine aquarium because they prefer water with a high calcium concentration and high pH; above all they thrive in water with high concentrations of nitrates and phosphates, due to poor management both of tank cleaning and of the inhabitants' diet (overfeeding).

It is not unusual to see them appear after a sharp drop in redox potential (ideal value between 200 and 250/280 mV) caused by neglect of the ecosystem: mechanical pre-filters left too long, irregular or insufficient water changes, lamps not replaced regularly or, with LED lighting, cheap lamps with a distorted spectrum, failure to clean the substrate, and so on. Overstocking, or keeping fish that turned out to be too big, can also mean an excessive organic load, unsustainable for a well-balanced ecosystem.

1. 1Too much food a wrong diet for the inhabitants
2. 2Overstocking or fish that turned out too big
3. 3Dirty substrate
4. 4Neglected lamps not replaced, or cheap with a distorted spectrum
5. 5Mechanical pre-filters left in too long
6. 6Water changes irregular or insufficient
7. 7Redox dropping sharply ideal between 200 and 250/280 mV

Behind a film there is often an excessive organic load.

Prevention

The best way to fight cyanobacteria is to prevent them, setting up ideal conditions so that they do not develop or at least regress until they disappear or stay confined to small areas of the aquarium; they are, after all, among the oldest organisms on the planet: getting rid of them is not easy, nor even necessary, if they are contained.

1. Aquascape the tank with good-quality (highly porous) live rock, so as to recreate the best environment for the nitrifying bacterial flora.
2. Keep nitrates and phosphates always and consistently at minimal concentrations: PO4 max 0.03, NO3 max 10 mg/l
3. Specific gravity not below 1.023 (ideally between 1.024 and 1.025)
4. Keep pH always above 8 and KH never below 8 dKH (sudden or excessive swings in KH can trigger the appearance of cyanobacteria)
5. Use and dose top-quality trace elements, paying particular attention to strontium, which corals like, but so do cyanobacteria!
6. Use a good skimmer suited to the size and population of the tank.
7. Use circulation pumps suited to the tank's volume, positioned so as not to create dead spots.
8. Use a lighting set-up made up of at least one third actinic lamps (the effect of the green and red LEDs fitted to some new-generation lamps is still debated)
9. Replace lamps on time, paying particular attention to how their light intensity decays; if in doubt, replacement every year is recommended.
10. Regular water changes suited to the tank's volume, made strictly with demineralised water and top-quality salt free of NO3 and PO4
11. Always and only use demineralised water with TDS close to 0 for top-ups
12. Scrupulous cleaning or replacement of all mechanical filter media in which bacterial flora has not become established.
13. Regular use of an excellent bacterial strain, both nitrifying and decomposing. This curbs a possible proliferation of "mutated", and therefore no longer useful, bacteria.
14. Fitting a UVC lamp that is always on, with the bulb replaced every year. (Here too opinions differ: some consider it essential in the fight against cyano, some consider it useless).

1. 1Porous live rock for the nitrifying bacterial flora
2. 2Nitrates and phosphates PO4 max 0.03, NO3 max 10 mg/l
3. 3Specific gravity not below 1.023, ideally 1.024-1.025
4. 4pH and KH pH above 8, KH never below 8 dKH
5. 5Quality trace elements watch the strontium
6. 6Suitable skimmer for size and population
7. 7Circulation pumps positioned to avoid dead spots
8. 8Actinic lamps at least a third of the set
9. 9Timely replacement if in doubt, every year
10. 10Regular water changes demineralised water, salt without NO3 and PO4
11. 11Top-ups demineralised water, TDS close to 0
12. 12Mechanical filters carefully cleaned or replaced
13. 13Bacterial strain nitrifying and decomposing
14. 14UVC lamp always on; opinions differ on its usefulness

Parameter	Value given by the author
Redox potential	ideally between 200 and 250/280 mV
Phosphates (PO4)	maximum 0.03
Nitrates (NO3)	maximum 10 mg/l
Specific gravity	not below 1.023, ideally between 1.024 and 1.025
pH	always above 8
KH	never below 8 dKH, without swings

These are values from the author's experience, not limits measured in a study.

If the problem persists

Despite all these precautions the problem may persist; then you can turn to "quicker" alternative systems that act on the effect but not on the cause (often there is no certain cause, so using certain products can solve the problem without the tank necessarily being overrun by cyano again).

1) Using antibiotics such as chloramphenicol, undoubtedly effective against cyano but, on the other hand, also fairly lethal to the nitrifying bacterial flora.

2) Hydrogen peroxide (H2O2): use water with 2 to 4% oxygen by weight (10 vol), so the commercial kind at 3.6% oxygen by weight is fine, at a dose of 1 ml for every 10 litres of tank (1.5 ml in the toughest cases); mix the hydrogen peroxide into about 1 litre of aquarium water and spray the resulting solution onto the film and not onto the invertebrates, even every day, until the cyano have completely regressed. "Warning: an excessive dose can be lethal to the entire ecosystem".

1. 1Hydrogen peroxide the dose is in the text, with its warnings
2. 2Dilute it in about 1 litre of tank water
3. 3Spray it on the film
4. 4Not on invertebrates the solution must not be sprayed on them

Like all quick systems, it acts on the effect, not on the cause.

3) Products such as Antired (Aquamedic) and Chemiclean have given excellent short-term results; alternatively you can use Cyano Clean together with A-Balance (Korallen-Zucht) for targeted biological control (although in this case it will take longer).

This report is the result of years of hands-on experience and of reading magazines and books, both scientific and from the hobby.

Article written by: Ciro Murino

Sources

1. Ford A.K., Bejarano S., Nugues M.M., Visser P.M., Albert S., Ferse S.C.A. (2018). Reefs under Siege: The Rise, Putative Drivers, and Consequences of Benthic Cyanobacterial Mats. Frontiers in Marine Science 5: 18.
2. Brocke H.J., Polerecky L., de Beer D., Weber M., Claudet J., Nugues M.M. (2015). Organic Matter Degradation Drives Benthic Cyanobacterial Mat Abundance on Caribbean Coral Reefs. PLOS ONE 10(5): e0125445.
3. Demoulin C., Lara Y., Cornet L., François C., Baurain D., Wilmotte A., Javaux E. (2019). Cyanobacteria evolution: Insight from the fossil record. Free Radical Biology and Medicine 140: 206-223.
4. NIOSH (2019). Pocket Guide to Chemical Hazards: Hydrogen peroxide. Centers for Disease Control and Prevention.
5. International Agency for Research on Cancer (1990). Chloramphenicol: Summary and Evaluation, Monograph Volume 50. IARC, via IPCS INCHEM.