How to Remove Red Slime Algae in a Saltwater Aquarium

Red Slime Coating Live Rock in Fish Tank

Shemp Abravanel / Flickr / CC by 2.0

Saltwater aquariums can be challenging in many aspects, but the appearance of "red slime" causes a chill down the spine of all marine aquarists. Coating substrate, live rock and even sessile invertebrates if left to flourish, red slime can feel like an uphill battle.

What Is "Red Slime?"

Not to be confused with red algae, "red slime" is technically cyanobacteria that contains a red pigment. Cyanobacteria is typically classified as "blue-green algae," but can come in various colors. These primitive algaes are thought to be early plants that first started surviving by photosynthesis, creating the oxygen-rich atmosphere we enjoy today.

How Does Red Slime Affect My Qquarium?

Like most unwanted algaes, red slime is mostly an eyesore. Depending on your tank conditions, red slime can flourish and blanket your tank rapidly. Sessile invertebrates, such as corals and clams, may get overtaken if the algae is not controlled.

Like other live plants and algae in aquariums, red slime is happy when the lights are on and create lots of oxygen through photosynthesis. However, when the lights go off, algae will use oxygen dissolved in the water to survive. If there is not enough oxygen to go around, your fish could be at risk for asphyxiation. If your kH, or alkalinity, isn't high enough, you may have a pH dip during the night as more carbon dioxide is produced and not being used for photosynthesis. Check your oxygen levels and kH levels at their lowest, right before the lights come on or the sun comes up, to make sure your levels are adequate.

How Do I Get Rid of Red Slime?

Like many other aquarium nuisances, red slime takes advantage of poor water quality environments and proliferates quickly. All it takes is one tiny cell to create a forest of red slime.

Phosphate, nitrate, and other dissolved organic compounds are the most common cause of red slime. Phosphates naturally occur in some water sources and are vital to building of hard coral homes. If you do not have corals in your tank and you are using a coral-specific saltwater mix, you can easily drop your phosphate levels by switching to a non-coral mix. Phosphates do not affect your fish's health.

Nitrate is the end stage of your nitrogen cycle. Although safe in lower concentrations, high nitrate levels can cause severe disease in fish, let alone cause an increase in algae growth. Nitrates are usually removed from your system by performing water changes. Getting behind on maintenance is the number one cause of red slime.

Like all photosynthesizing life, cyanobacteria relies on a light source to grow. Red slime, in particular, likes 560 to 620 nm spectrum. By reducing your light levels, but not so drastically as to hurt your other photosynthesizers, like the zooxanthellae in corals, you can reduce your red slime load.

Although less likely, hypersalinity can also cause an increase in algae growth. Hypersalinity usually happens when you top off your evaporation with saltwater. Although it may feel completely against your reasoning, saltwater systems need to be topped off with fresh water. When evaporation occurs, the salt molecules remain behind, causing increased salinity. Topping off with freshwater sets this back to the correct salinity. Test your water salinity multiple times a week with a simple refractometer.

Some antibiotics may be effective, but they are not controlled and will wipe out biological filtration. They will certainly only act as a bandaid and your algae will return if you have other predisposing factors that are not addressed.

Once you have figured out what parameters have promoted red slime growth, you can siphon out as much as possible. Using a tiny rigid airline suction hose will get into the tight spaces and crevices. Remember, it only takes one cell to grow a blanket!


Ensure that you safely dispose of any collected cyanobacteria. This substance, depending on its species and composition, can be toxic to other animals, namely dogs. Do not pile it up in the backyard where your kids or dog can access it.

How Do I Prevent Red Slime?

The best way to prevent red slime is to keep up with your regular saltwater aquarium maintenance regimen. A few missed water changes can give red slime a chance to take hold and overrun your system.

Use a saltwater mix that is best for your system. If you do not have hard corals, you may not need a coral-specific mix. This will decrease the phosphate load in your tank easily.

Make sure your lighting is appropriate for your tank. This will depend on your other photosynthesizers, mainly corals. Don't dump antibiotics into your tank. The fix won't last and you'll be looking at starting your nitrogen cycle from scratch. Once you master the predisposing factors that breed red slime, you will be rid of it for good.