Red Slime Algae is the common name for an aquarium troublemaker otherwise known as red cyanobacteria. Since the term cyanobacteria sounds like either a bizarre Crayola Crayon color or the bacterial strain in a Michael Crichton novel (how’s that for a dated reference), the more popular term red slime algae tends to fit quite well.
It does have a reddish tint (which can appear on the brown or purple ends of the light spectrum, depending on the lighting in your tank and thickness of the growth) and a slimy look about it, which is likely how it got the name.
If the shoe fits, wear it.
Red slime algae are universally hated by saltwater aquarists, mostly because it:
- Grows even though it is unwanted
- Can sometimes be difficult to control
- Creates extra work
- Is generally thought to be ugly
- Seen as a sign of an ‘out-of-control’ tank
How’s that for a resume?
Red slime algae affect many tanks–this is a prominent reef display at a public aquarium
However, unlike the dreaded saltwater ich, marine velvet, rapid tissue necrosis, old tank syndrome, and other aquarium problems that have been known to kill or even wipe out an entire system, all red slime algae are known for is…well…being ugly and slimy.
With that said, a red slime algae outbreak in your tank is nothing to laugh about.
What causes a red slime algae outbreak?
I’m not sure that we, as a saltwater aquarium or scientific community, have figured out exactly what the recipe is for a red slime algae outbreak, but here are a few important elements that seem to show up time and time again:
This tank has a little bit of everything going wrong: cyanobacteria, bubble algae and aiptasia
Underlying Mechanisms:
Red slime algae (cyanobacteria) outbreaks are closely tied to the stability and maturity of your aquarium’s ecosystem. In new tanks, nutrient cycling is still stabilizing, and the microbial community is in early succession. This means there are fewer established competitors—such as beneficial bacteria, microfauna, and algae—that would otherwise keep cyanobacteria in check. Excess nutrients (especially nitrates and phosphates), combined with available surfaces and light, create ideal conditions for cyanobacteria to thrive. As tanks mature, microbial diversity increases, nutrient processing becomes more efficient, and outbreaks become less likely.
Disruptions such as major cleanings, medication, or sudden changes in bioload can destabilize even mature tanks, resetting the balance and making them temporarily vulnerable to cyanobacteria blooms. Recognizing whether an outbreak is part of normal early tank instability or a sign of a deeper, ongoing imbalance is important: in new tanks, patience and gradual correction are often the best approach, while persistent outbreaks in mature tanks may indicate chronic nutrient or flow issues.
Age of the aquarium
No, not the Age of Aquarius, but the age of your aquarium. The outbreaks I have encountered mostly occur in tanks that are about 0-6 months in age. I have encountered this personally, as well as someone providing advice to people who have emailed me over the years because of this blog.
My hypothesis is that red slime algae thrive in that new tank environment. Over time, the inhabitants (micro and macrofauna) of an established saltwater aquarium help to keep the red slime algae in check—but in the early stages of saltwater aquarium maturity, red slime algae seems to get a foothold much more easily.
Three things cyanobacteria need to grow
Red slime algae is a photosynthetic organism that needs three things to grow:
- light for photosynthesis
- nutrients to fuel growth
- a structure on which to grow.
It tends to get a foothold in an area of the tank with low flow.
11 Ways to Fight Red Slime Algae
If you want to combat the growth of red slime algae in your tank, you have to hit it where it hurts:
- Light
- Nutrients
- Structure
- Kill it chemically
How to Prioritize Your Battle Against Red Slime Algae
Before diving into specific interventions, it’s important to approach cyanobacteria outbreaks with a clear strategy. Start by addressing root causes—nutrients, flow, and light—before escalating to chemical treatments. Quick fixes may provide temporary relief, but lasting success depends on correcting the underlying imbalances that allow cyanobacteria to thrive.
Decision Framework:
- Assess and reduce excess nutrients (especially from food and detritus).
- Improve water flow to eliminate dead spots.
- Evaluate and optimize lighting (spectrum and duration).
- Use chemical treatments only as a last resort or for severe outbreaks, and always in conjunction with root cause correction.
Common Mistakes to Avoid:
- Overfeeding or using high-phosphate foods
- Over-cleaning biological media, disrupting beneficial bacteria
- Relying solely on chemical treatments without addressing causes
- Reducing nutrients or light too aggressively, harming livestock
When to Use Patience:
In new tanks, some instability is normal. If you’re already addressing nutrients, flow, and light, sometimes the best course is to allow the tank to mature while maintaining good husbandry. Escalate to more aggressive interventions only if the outbreak is persistent or worsening.
1) Perform partial water changes
Nitrates, phosphates, and silicates are waste products in your aquarium water that can act as fertilizer for red slime algae and fuel its growth. To fight red slime algae, you should first take your fight to the water quality and remove nitrates, phosphates, or silicates. The easiest way to do this is with partial water changes. While you do your water changes, give the red slime algae a blast with a turkey baster or a powerhead. You want to dislodge it from the rocks and physically remove it from the aquarium water. This is a pain. It feels like thankless grunt work, but it is a necessary step to regain control of your tank.
2) Manage your feeding: food quality and quantity
What are you feeding your fish and corals? Both the type and amount of food you use can significantly affect nutrient levels in your tank. Some foods are naturally high in phosphates or silicates, which can pollute your water and fuel cyanobacteria growth. Check food labels and research ingredients online to identify potential nutrient sources. If you use frozen foods, like frozen mysid shrimp or brine shrimp, rinse the cubes in tap water with a strainer before adding them to your tank to reduce excess nutrients.
Just as important is the quantity of food. Overfeeding is a common mistake—uneaten food breaks down and releases nutrients that feed problem algae. While battling red slime algae, reduce the amount of food you add to your tank and feed only what your livestock will consume in a few minutes.
3) Add a chemical media or media reactor to combat cyanobacteria
If phosphates, silicates, and nitrates remain a problem, you can add a chemical media like GFO (granular ferric oxide) or biopellets to help. GFO binds phosphate, removing it from the water, while biopellets provide a carbon source for bacteria that consume nitrate and phosphate. However, these media should be introduced gradually—rapid reductions in nutrients can stress corals or destabilize your tank. Overuse of GFO can lead to low phosphate levels, causing coral bleaching or poor growth, while excessive biopellet use may trigger bacterial blooms. Always monitor nutrient levels and observe your livestock for signs of stress when using chemical media, and adjust dosages as needed.
4) Keep the water flow high
Red slime algae likes to get a foothold in areas of low flow. Low flow allows detritus to settle and accumulate, creating localized nutrient spikes that favor cyanobacteria growth. To prevent this, move powerheads or add additional pumps to eliminate dead spots—areas where debris collects or water movement is minimal. Practical ways to identify dead spots include observing where detritus settles or using food dye or fine particles to visualize flow patterns. Target these areas for increased circulation to help keep red slime algae in check.
5) Change old light bulbs (if you have them)
The first thing to look for, from a lighting perspective, is whether you have fresh bulbs. As aquarium light bulbs age, their spectrum shifts toward the red end, which can favor nuisance algae and cyanobacteria. Sometimes, changing the lightbulb can be enough to correct the problem, if things are otherwise stable.
Make sure to keep good records of when you change your light bulbs and schedule a reminder so that you won’t forget. Of course, you don’t need to do this for quite some time if you buy LED lights.
Why does spectrum shift matter?
As bulbs age, the light they emit changes, often producing more wavelengths that cyanobacteria and nuisance algae can use more efficiently than corals. This gives red slime algae a competitive edge. Replacing old bulbs helps maintain a spectrum that favors your desired livestock.
6) Reduce light
As mentioned earlier, red slime algae need light to grow, so in certain circumstances, you may want to shut off or reduce the light over your tank for a couple of days while you wage war on the red slime algae invader.
Adjusting the light over your tank to combat the red slime algae can be a tricky endeavor if you have other (more important) photosynthetic organisms in your tank, like corals. For the most part, if your corals are otherwise healthy, you can reduce the photoperiod (duration) or slightly lower the intensity for a few days to try to get a jump-start on your red slime algae clean up. Avoid drastic reductions that could stress your corals—short, temporary adjustments are usually safe for healthy livestock.
7) Keep filter media clean
If you have a mechanical filter, like a filter sock inside the sump or a pre-filter sponge on the intake of your return pump, be sure to keep it free of debris, detritus, and leftover food. In one way, that’s a sign that the filter is working; on the other hand, any matter that is decaying on that sponge is being blown back into your tank and could be causing nutrient levels to rise.
Note: Clean mechanical media regularly, but avoid over-cleaning or replacing all filter media at once, as this can remove beneficial bacteria and destabilize your biofilter.
8) Maintain the proper pH
Keeping your water parameters in line, especially pH, can help you fight red slime algae. Cyanobacteria outbreaks are often associated with lower pH environments. Keeping a naturally high pH (8.1-8.4) can help create an environment ideally suited for your corals and less ideally suited for the red slime.
9) Time
Some of the most dedicated hobbyists who stay on top of things will email me saying, I’m doing everything you said, and I am still having problems.
I wish I had a perfect answer for this—and some science to back it up—but what I usually say in that instance is to just keep with it. Time and tank maturity are factors. I’m not sure exactly what happens to turn the cyanobacteria imbalance into balance, but I can tell you that eventually, it happens.
One day, you win the battle and the red slime algae fade away. So stay on it, stay positive, and don’t let up.
10) Chemical Additives
Two chemical additives to treat red cyanobacteria are Chemiclean and Red Cyano Rx. These products do a great job of getting rid of red cyanobacteria in just a matter of days. If you are going to use them, increase the oxygen flow in your tank with an air pump or more flow, because these chemicals will make the oxygen level in your tank go down. You will also want to turn your protein skimmer off.
What is chemiclean?
A product from Boyd Enterprises that cleans red, black, blue-green, and methane (bubble) producing cyanobacteria in marine aquariums.
It is completely safe for all fish, corals, invertebrates, and nitrifying bacteria in reef systems. It’s simple, quick, and easy to use. Chemiclean works within 48 hours oxidizing trapped organic sludge and promotes an ideal enzyme balance. Chemiclean will clean stains from red cyanobacteria in aquariums.
The only thing that isn’t safe is the possible drop in oxygen. So be sure to increase those levels to help keep your fish safe.
Important Note:
Chemical treatments should be used as a last resort or for severe outbreaks, and always in conjunction with efforts to address the root causes (nutrients, flow, light). If underlying issues are not corrected, cyanobacteria will often return after treatment, leading to cycles of recurrence and frustration. Use these products judiciously and focus on long-term prevention.
For more information, you may want to check out these videos:
This is how Reefbum fought his red slime algae: