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

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.

  10. avatar

    I also have the problem with the slime and smell, my question is would it attract mosquito, as my pumps not working very well.

  11. avatar

    Hello Angie, Are you referring to an outdoor pond or an indoor aquarium? Mosquitoes will be attracted to standing water to lay their eggs but that isn’t usually an issue on indoor aquariums. If the “slime” you are seeing is accompanied by a bad smell, it is likely not the cyanobacteria referred to in this article but is rather likely due to a combination of inadequate filtration, overcrowding and/or overfeeding. It is difficult for me to say without knowing more about the pond/aquarium you have, how large it is, what is in it, the water chemistry, etc.. Fresh activated carbon and water changes will help you control those symptoms in the meantime while you diagnose the cause of them.

  12. avatar

    Is this cyanobacteria slime the same translucent white stuff that sometimes accumulates on suction cups and rubber parts of heaters in new aquariums?

    I set up an aquarium two weeks ago, and about a week later started seeing that while slime on my heater’s rubber parts. I clean it off every couple of days, but it comes back in about two days. There are quite a few posts online from people who have the same thing, so apparently it’s common. I’d appreciate any help you can offer.

  13. avatar

    Hello Lou, Cyano wouldn’t be white. Black, green, blue-green or red, but not white. I would say that you are probably experiencing what is commonly known as “New Tank Syndrome” or you might be seeing a fungus growing on those surfaces. I would expect that more on leftover food than on equipment but if there are spores or too many nutrients in the water, it is possible. If you would like to send us some photos of this “white stuff” you are seeing with some basic info about your tank (at least its size, a list of any livestock in it, water parameters like pH, temperature, Ammonia, Nitrite and Nitrate levels to start with), someone here can take a look at it and try to diagnose it more accurately for you. You can send those photos as attachments to fish@thatpetplace.com.

  14. avatar

    There is only one permanent fix to this problem. Sufficient aeration of the water.

    The ONLY time EVER that I’ve seen green slime algae become a problem in ANY tank regardless of water quality is when there is not enough dissolved oxygen in the water. With sufficient aeration, it goes away completely on it’s own.

    It makes sense, cyanobacteria require a low-oxygen environment to live. If anyone has a bead on another common and plausible cause, I’d be interested to hear it. But I’ve never encountered it before. The advice online is good… water quality… etc… etc. But when I see it, I’ll believe it.

    The fish seem to survive just fine in these low-oxygen tanks covered in green slime, but mine seemed much more lively once I fixed the problem.

  15. avatar

    Hello atsaunders, Low oxygen may certainly contribute to the problem but I would disagree that it is the primary cause and aeration is the only solution. Other issues like poor spectrum lighting and water chemistry certainly feed into it as well. The reason you may see it in more heavily aerated tanks is because the cyano develops air pockets under the mat which make it semi-buoyant. Aeration in the tank will help dislodge the mats and allow it to be picked up by the filtration easier. This is why it is also often seen in areas of low flow; with sufficient flow, it will be disturbed enough to become dislodged and filtered. Without eliminating the cause – be it low oyxygen or poor light quality or excess nutrients – the cyano will always return.

    For a few quick references from scientific sources on the causes of cyanobacteria (particularly in natural environments, you can check out these sources from the Environmental Protection Agency, World Health Organization, Centers for Disease Control and Prevention, and the Australian Department of the Environment.

  16. avatar

    Hello I would like to know if anyone knows what this clear gel like substance is in my fish tank. I recently cleaned everything in it and scrubbbed everythung really well. New grave, new sand and a bubble wand. I scrubbed my decirations really well. I have a picture that I can send to anyone that’s wolling to help me figure out what it is.

  17. avatar

    Hello John, You can email us at fish@thatpetplace.com and someone here can try to help you with your tank. Please include as much information about your tank as possible – size, types of livestock in it, water paramteres (actual test values of pH, Ammonia, Nitrite, Nitrate at least as well as Salinity if it is a saltwater tank), how long the tank has been set up, what maintenance you regularly do, etc.. The more information you can give us and the better your photos are, the more likely we’ll be able to help you.

  18. avatar

    I have a 5 plecos in a 46 gallon tank. Six inch sail fin, two 5 inch green phantoms, two 3 inch royal, and three 1 1/2 albinos. They eat romaine lettuce, carrots and cucumber. I do a 50% weekly water change. If I don’t rinse off the filter media every few days, the 2 Marineland 400 Emperor HOB filters will clog with a huge amount of brown gelatinous sludge. The HOB filters then need to be taken apart and cleaned biweekly. The tank also has clown loaches, mollies, corys, and angels, but I don’t they contribute as much waste as the plecos. A whole cucumber, carrot, or lettuce heart is rapidly converted to waste and then sludge, which winds up in the filter. A garden hose is the best way to clean the filters.

  19. avatar

    Hi Tom, It sounds like the “slime” you are seeing is more a result of overcrowding and waste rather than the cyanobacteria discussed here. That is a lot of large, high-waste fish for a tank that small even before knowing the sizes and quantities of the other fish you mentioned. I would recommend upgrading the size of your aquarium or cutting down on the number of plecos in it, which will also allow you to feed less.

Leave a Reply

Your email address will not be published.

About marinebioblog

Read other posts by

Marinebioblog is the post name of That Fish Place - That Pet Place's aquatics and aquarium experts. Contact them through the links here or leave your comments below.