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Aquarium Slime: What is it and what to do about it?

Please welcome back Desiree Leonard with another “What’s this and What do I do?” article.

This frequently encountered problem is Cyanobacteria or “Slime Algae”.

The name “slime algae” is a misnomer. Because Cyanobacteria are photosynthetic and aquatic, they are often called “blue-green algae”. In reality they are NOT algae, but something more in between algae and bacteria. Cyanobacteria are bacteria that manufacture their own food and live in colonies — large enough for you to see them! It’s these colonies that cause trouble for aquarists. They are not necessarily blue-green but can be black, green, blue green, and the familiar dark red sheets covering many surfaces in an aquarium.

The first thing aquarists who find an unwanted colony of cyanobacteria in their aquarium want to know is how to get rid of it. Well, this is where it gets tricky. To eradicate the problem – the particular trigger for the cyano bloom must be identified and treated. Not every bloom is in response to the same trigger and while throwing a chemical at the problem will perhaps clear it up temporarily, it will come back, and it will be worse. (More on this later.)

As with all types of algae, any uncontrolled growth indicates an imbalanced system. An imbalance in one or both of two main triggers can set off a cyano bloom.

• DOC – Dissolved Organic Carbon is a food source of the bacterial side of the bacteria-algae. Sources of dissolved carbon include: fish slime, algae, bacteria, digested/uneaten food, metabolic waste, live food, some aquarium additives etc.
• Lighting – The food source for the algal side of the bacteria-algae is light. Light bulb spectra shift to red as they age, resulting more favorable conditions for photosynthesis to take place more vigorously.
Note: It is said that slime is caused from phosphates and silicates in the water. It’s true that these 2 elements will certainly grow algae of all sorts, but if removed will not reduce or remove a slime problem.

Okay – so what DOES remove the problem?

• Control your DOC. This is best done by frequent water changes, good water movement (power heads and closed system circulation) and (this is important!) a good protein skimmer. An undersized or ineffective protein skimmer, high waste loads, or a combination thereof will increase the dissolved carbon level. As a rule of thumb for skimmers; buy one that is rated for at least twice the size of your tank. It may take some adjusting but a properly functioning skimmer can remove ½ cup of thick organic scum from a tank a day.
• Use an RO/DI filtering system (Reverse Osmosis/Deionization) for water changes whenever possible. This eliminates adding DOC into your tank via tap water.
• Add more lighting or change your bulbs. Change bulbs at least once every 9 to 12 months, don’t wait till they burn out. To be more cost effective, you can stagger your replacements rather than replacing them all at once, but if the slime persists you may have to go all out and do full replacement.
• Watch what you feed. Feed once a day. If you wish to feed twice, simply split the amount in half – don’t feed twice as much food. If you feed grocery store bought seafood or are making your own foods, rinse all foods thoroughly as seafood sold for human consumption is treated with phosphates and preservatives to keep it fresher longer. (It’s true!) Avoid flake foods, these dissolve too fast – pellets and crisps are much better and more palatable.
• If you aquarium is freshwater, the above treatments still apply, but a protein skimmer is not used. Water circulation, frequent water changes, extra charcoal filtration and changing lights all will be effective controls.
Disclaimer: I am in no way saying that if you have a slime outbreak, that you are a bad aquarist and your water is swill. Even in the best kept tanks there are still cyanobacteria. You will, in fact, see outbreaks in systems which are free of phosphate and silicate; they also have new halides, actinics and great water flow. There is always another factor – vitamin supplements, liquid foods, and other additives can add the organics that can trigger a cyano explosion. Look for anything different you are doing and stop doing it.

Right – that covers the long term, not so easy fix. But for those who still want a quick fix, there are products that are available to help remedy the problem. BUT – if the underlying issue is not addressed, don’t say I didn’t warn you…..

• Cyano is a gram negative (thin cell membrane) bacteria, much like most bacteria in the aquatic environment. A dose of Erythromycin will knock out the colony of slime quite quickly. However, since the nitrifying bacteria you need in your tank are gram negative as well, they will be affected also, either being killed or severely damaged. This treatment is more advisable in freshwater aquaria, but only with careful attention paid to water quality while treating. Like all antibiotics, if dosed frequently the cyano will develop a resistance.
Chemi-Clean by Boyd Enterprises and Red Slime Control by Blue Life are highly effective reef safe treatments for slime. These are non-antibiotic formulations and will do less damage to your biological filter. If used frequently however, there is still a chance of the cyano developing a resistance.
I hope this info is helpful in your endeavors to keep a slime free tank. For more and more thorough information, check out these links! Happy Fish keeping!




  1. avatar

    I also have recently had slime, but colourless, mostly in my sump. It is almost (clear) mucuslike. Slippery slimy, but again, colourless (and odorless). What might this be?

  2. avatar

    It could just be a coat of naturally occurring bacteria, probably nothing to worry about. May also be residual material from fish and inverts in the tank, also not anything that should harm the tank. Cyanobacteria is dark and unsightly, easily recognizable.

  3. avatar

    Hi, Thanks for the reply. Probably not dangerous, but very annoying, because it cloggs up my pumps and skimmer. Anyway, by coincidence, I have had to take out EVERYTHING due to a leakage in a seal. Have been able to clean sandbed and more than 50% waterchange. We’ll see how it goes, now we are of on a holiday. Thanks again.

  4. avatar

    I have the same problem as Hugo slippery slimmy colorless, odorless slime what might this be? Can anybody tell me can that harm my betta and how can I get rid of it?

  5. avatar

    Selena, this is most likely just a bacterial slime, and most likely harmless. Bacteria is going to grow on all surfaces of your tank, and is nothing to worry about it.

  6. avatar

    never had problems in 5 years of keeping Oranda but just few weeks have sudden appearance of masses of slime (like frogspawn) everywhere. Cleaned everything, scrubbed stones, tank etc. but a week later it’s back and worse. My fish are staying at the top of the water and looking unhappy! Can someone please help as I’m getting desperate?

  7. avatar

    Hello Janet, What are the water parameters (pH, Ammonia, Nitrite, Nitrate, temperature at least), how large is the tank, and how many fish are in it? How often do you do water changes and how much do you change? If you have any photos of the mass you are seeing, you can email them with information about your tank to fish@thatpetplace.com and if you would like to speak with someone about your tank issue, you can call us at 717-299-5691.

  8. avatar

    Hello Eileen, thanks for your response. So sorry I have not replied before now but I have been away and have since been quite poorly. I am not sure of water parameters exactly but check with the plastic sticks regularly and pH,Ammonia levels always ok. I change the water about once a month – normally leave about a third of the tank water taken from the top as its cleaner. I only have two fish both about 4″ long and the tank is 27L. I am still getting this problem. The slime is clear to pale brown, and the tank really smells bad which it never did before. Any ideas would be welcome, thank you!

  9. avatar

    Hello Janet, 27 liters (or about 7 gallons) is very small for any 4-inch fish, much less for goldfish that you mentioned back in December. It sounds like it might be a water quality issue. I would recommend having your water tested for pH, Ammonia, Nitrite, and Nitrate to start with. You mention “plastic sticks”. If you are referring to test strips, those are notoriously inaccurate and have a very short shelf life. Having the actual values of these tests would be helpful. With fish that large in such a small tank, it sounds like the filter isn’t able to keep up with the waste and that would be causing the smell and slime. Even one goldfish at that size should have at least a well-filtered 30 gallon (100-125 liter) tank.

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