Getting Rid Of Red Slime Algae By Addressing The Root Cause

Saltwater tanks require a lot of time and knowledge to maintain properly. With all the money and effort you put into your tank, it would be nice to sit back and enjoy watching your fish through clear glass, but the presence of red slime algae can ruin the results of your efforts. With a little bit of experimenting, you can get rid of that red slime algae and have your tank looking crystal clear by determining the underlying cause of the problem.

What Is Red Slime Algae?

First of all, red slime algae isn't really an algae, it's a cyanobacteria, a type of bacteria that performs photosynthesis. It will start out as a small patch in your tank and spread to cover the walls and coral. There are aquarium products you can purchase to clear the tank of the algae but many of these products contain antibiotics that will weaken the biological filter base of the tank, so it is best for the overall health of your tank to avoid them.

In order for algae to grow they require light and nutrients. Getting rid of red slime algae permanently will involve addressing the underlying problem by removing these requirements from the environment.

Possible Cause: Lighting

Red slime algae thrive in the 665 to 680 nanometer wavelengths (red light). They also prefer lower temperature light sources and longer periods of light.


Make sure that the light on your saltwater tank is designed for aquarium use and that it is a higher temperature light source, above 10,000k. Also, if you have an old metal halide light, it will give off more light in the red spectrum.  This encourages the growth of red slime algae. Replacing your light every year will ensure its efficiency. Also, turn the light off at night. It should only be running for 8 to 9 hours a day.

Possible Cause: High Levels of Phosphates

One of the primary nutrient sources for red slime algae is phosphates. Phosphates are usually introduced into the tank through the use of tap water and sea salt mixes that contain high concentrations of this nutrient, but they can also be added through the use of activated carbon, buffers, and fish food.


When adding new water to the tank, use water that has been filtered through a reverse osmosis/deionization (RO/DI) filter unit. Your local fish store may sell this type of water. Also, make sure that other aquarium supplies you purchase such as sea salt mix, activated carbon, and buffers contain low levels of phosphates.

Possible Cause: High Levels of Nitrates

Nitrates can be introduced to the tank through the same methods as phosphates but their levels can also rise when excess dissolved organic compounds are allowed to accumulate in the tank. The introduction of new live rock can also add excess nutrients to the tank.

When testing for nitrates in the presence of red slime algae, low or normal levels of nitrates could be deceiving. The red slime algae are feeding on the nitrates, removing them from the environment, so the actual levels of nitrates could be higher than what is indicated by the test results. Solution:

Proper aquarium maintenance is necessary to limit levels of nitrates. This should including checking and changing the filter regularly, cleaning the tank, and performing partial water changes. Also, make sure you are not overfeeding your fish. Visit your local fish store to purchase proper aquarium supplies to limit nitrate levels. For instance, protein skimmers can remove nitrates whereas wet/dry trickle filters contain bio balls that encourage the production of nitrates. When adding new live rock to the tank, make sure to cure it properly.

Take It Slow

When correcting your red slime algae problem, only try one of these solutions at a time. This will help you detect the actual underlying cause to the excessive growth of this organism. As you change the various factors that could be contributing to the problem, you can also manually remove the algae as a temporary solution or add an organism to the tank that feeds on the red slime algae such as the Left-Handed or Dwarf Zebra Hermit Crab.

With a little effort and patience, you should be able to see the red slime algae levels decrease. Once you figure out the root of the problem, it will be easier to keep this organism at bay and enjoy watching your fish through a clear tank. Contact an aquarium supplies provider, such as Congressional Aquarium, to buy the right supplies.