How to Cure Red Slime Algae in Marine Aquariums

Three Parts:Removing Red Slime Algae From an AquariumModifying Your Aquarium to Prevent Future Algae ProblemsImproving Your Water Quality

Red slime algae is a type of cyanobacteria, which behaves like both bacteria and algae.[1] Red slime algae looks reddish-brown in appearance. It tends to show up slowly in small clumps/patches before quickly spreading and covering every part of your aquarium.[2] If you've got red slime algae in your aquarium, you'll need to take quick steps to remove the immediate problem and take precautions to prevent future growth.

Part 1
Removing Red Slime Algae From an Aquarium

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    Perform a partial water exchange. If your tank is harboring red slime algae, the first step is to replace the nutrient-rich water in your aquarium with clean saltwater. You can do this by performing a partial water exchange.[3]
    • Aim to remove and replace about 10% to 20% of the tank's water. In a 10 gallon tank, that means about one or two gallons, which should be sufficient to ensure healthy water.[4]
    • Remove as much organic material and detritus from the tank's substrate as possible. Using a vacuum tube can be helpful here at removing debris from the gravel or sand at the bottom of the tank.
    • Make sure you also scrape away any red slime algae that's clinging to the underwater surfaces inside your tank.
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    Remove settled organic debris from the tank's substrate. Even if you've performed a partial water change, the tank may still be a breeding ground for red slime algae. Adding clean water isn't always enough, because the nutrients that algae feed off of are often lurking elsewhere in the aquarium, especially amid the gravel or sand at the bottom of your tank.[5]
    • If you can afford it, an under-gravel filter might be a great way to constantly remove excess debris from the gravel or sand.
    • A gravel vacuum can be useful for sucking out debris that settles in the gravel.
    • You can also use a siphon to clean out the substrate of your aquarium.[6] You can find siphons at many pet stores or by searching online.
    • Introducing orange-spotted sleepers and pistol shrimp to your aquarium can be helpful if you use sand as a substrate instead of gravel. These creatures tend to sift through the bottom of the aquarium and stir up the red slime algae, allowing your tank's filters to catch and extract it all out of the water.
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    Scrape off excess red slime algae clinging to your tank. If your tank is experiencing algal growth, it's important to take care of the water in your tank. However, algae that's already clinging to the tank can end up growing and blooming more, even if you're treating the water. Part of any red slime algae eradication should involve scraping away the algae that is clinging to your tank's walls and other internal surfaces.[7]
    • You can purchase a specialized scraper, like a Mag-Float, Algae Magnet Cleaner, or Kent Pro scraper.
    • These scrapers are available at many pet stores. If you cannot find one at your local pet store you can purchase one through an online retailer.
    • You can also try using the edge of an old credit card in a pinch, as this can help manually remove caked-on algae the same way a scraper would.
  4. Image titled Cure Red Slime Algae in Marine Aquariums Step 4
    Clean your filter. A dirty filter is often the culprit behind algal blooms in an aquarium. If you're experiencing red slime algae, make sure you clean the filter as part of your algae eradication efforts.[8]
    • The media inside mechanical filters will need to be cleaned whenever they become dirty, and once the residue no longer washes off the filter it will need to be replaced.
    • If your tank has a dirty filter, the organic waste trapped inside could be releasing ammonia, nitrate, nitrite, and phosphate into your tank's water.
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    Try using a protein skimmer in your tank. One of the biggest causes of red slime algae is the presence of organic debris (like left over food, fish waste, etc.) in the water column. Protein skimmers may be more effective than a regular filter at removing these materials from the tank's water.[9]
    • Protein skimmers pull out organic waste and will not allow that waste (or its chemical byproducts) to be released back into the tank's water, which may be a problem with other mechanical filters.[10]
    • Large external skimmers are the most effective at removing organic waste, but they must be properly adjusted to your tank. If they are not adjusted, they may leak a substantial amount of water onto the floor of your home.
    • You can buy a protein skimmer at many pet stores or through online retailers.

Part 2
Modifying Your Aquarium to Prevent Future Algae Problems

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    Limit how much you feed your fish. Excess food is one of the most common sources of elevated phosphate in your aquarium. As that phosphate permeates the water, it creates an ideal environment for algae to bloom, which can make a mess out of your tank.[11]
    • It's important to watch how long it takes your fish to consume the food you give them.
    • If a few minutes have passed and there's still excess food that's not being eaten, that food will settle in the substrate of your aquarium.
    • Try feeding your fish one larger meal each day, or offer smaller portions if you want to continue feeding your fish multiple times each day.[12]
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    Add aquatic plants to your aquarium. Because plants compete with algae for resources like light and nutrients in the water, they can help reduce or prevent algal blooms in an aquarium.[13] As these plants take in nutrients like phosphorous and carbon dioxide, they produce oxygen, ensuring a cleaner, healthier environment.[14]
    • Sea grass and mangrove plants make excellent aquarium additions.[15]
    • Certain macro algae species can also be good at oxygenating marine aquariums. These algae are healthy and helpful additions to an aquarium, and can help displace red slime algae.
    • Some good macro algae species to try planting include caulerpa sertulariodes, calupera asmeadia, and caulerpa mexicana.[16]
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    Limit overhead light exposure to your aquarium. While light is necessary for most aquatic plants to survive, too much light exposure inside an aquarium can create an environment that allows algal blooms.[17] This usually only becomes a problem with prolonged and close-range light exposure, and certain precautions can be taken to ensure a healthy tank.
    • Tanks should not be exposed to lights for more than 6 to 10 hours each day for ornamental aquarium setups, or 10 to 14 hours each day for planted aquariums.[18]
    • Try keeping lights on a timer if you're having trouble regulating the light cycle.
    • Make sure you change your light bulbs at least once every year. As lightbulbs age they lose their spectrum and intensity, which may promote algal blooms.

Part 3
Improving Your Water Quality

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    Check your tap water. If your aquarium seems to keep spawning new algal blooms no matter how much you clean the tank and limit food, it could be a problem with your tap water. It's generally a good idea to test your tap water to see if it may be contributing to the problem or causing it directly.[19]
    • You can test your water at home by buying a water quality test kit. These can easily be found online and may be available in your local pet store.
    • Phosphate is commonly added to municipal water supplies to help prevent pipe corrosion. It can also show up in well water if it leaches through the ground from yard fertilizers.[20]
    • Elevated levels of phosphates in your tap water can promote algal blooms in your aquarium.[21]
    • If your water is the problem, you can purchase a reverse osmosis water filter or other tap water filtration system. These are available at many home improvement stores or online.
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    Improve the movement of water in your tank. It's important to control the movement of water through your tank to ensure a healthy environment. Water flow will help keep the tank more oxygenated, but water that flows too fast can also have some negative consequences.[22]
    • If water does not flow quickly enough, it can become stagnant and less oxygenated.
    • When water flows too quickly, it can sweep away food into the tank's filtration system before fish have had a chance to finish eating it. Once inside the filter, this food will decompose and release additional phosphorous.
    • A powerhead or wave-making device can help improve water quality by increasing the rate of movement within your tank.[23]
    • Talk to an aquarium service expert about determining the proper flow rate for the size and type of aquarium you are managing. You can find experts online or by searching in your local phone book.
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    Perform weekly partial-water exchanges. Performing regular partial-water exchanges helps ensure a healthy environment. It's not so shocking to fish as having an entirely new tank full of water, but it's enough to keep the levels of nitrates and phosphorous down and remove dissolved organic material by displacing old water with cleaner, more-oxygenated water.[24]
    • Remember to only add salinated water to your marine aquarium. Aquarium salt can be purchased at most pet stores or through online retailers and mixed into your water at home.
    • Pour the new seawater into your tank very slowly. A rush of incoming water can frighten fish and could disturb the ecosystem.
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    Test your tank's pH. Part of keeping an algae-free aquarium should involve regulating your tank's pH on a regular basis. A slightly basic environment (meaning a slightly higher pH) can help prevent red slime algae from growing rapidly/aggressively once it has shown up in your tank.[25]
    • Most marine tanks should have a pH somewhere between 8.1 and 8.4.
    • You can adjust the pH with buffers or kalkwasser as needed.
    • You can buy a water pH testing kit at most pet stores that sell aquarium supplies. You can also purchase them through most online retailers.
    • Check the pH every time you perform a partial water exchange and adjust the water's pH accordingly.
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    Clean your filter regularly. If you do a partial water change but don't clean your filter, the clean water will just be running through a dirty filter. This will reintroduce a lot of the nutrients and organic debris that you've worked so hard to remove from the tank.[26]
    • You should clean your filter every time you do a partial water change.
    • Some tank owners rinse the filter's media in a bucket of wastewater that's been removed from the tank so that the healthy bacteria on the media is not lost.
    • Other tank owners may prefer running the filter's media under a faucet to blast away debris and detritus, though this may damage the bacterial colonies.
    • Whichever method you choose, proceed with caution so that the bacterial colonies are protected, but make sure you remove all of the organic detritus/debris (or as much as possible).


  • Use a small vacuum tube to suck out red slime algae and any debris settled in your tank's substrate.
  • Remove slimy rocks and scrub them with a stiff brush under running tap water or replace them to reduce the presence of red slime algae.


  • Make sure that you only add salinated water to your marine aquarium. Adding freshwater could shock or kill the plants and animals living in your tank.

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Categories: Aquariums