Brown Algae in Fish Tank

Medically Reviewed by Kathleen Claussen, DVM on July 15, 2023
4 min read

Brown algae is a common problem in many fish tanks, especially those that have been recently set up. It’s not dangerous to fish but can turn your beautiful aquarium into an ugly mess. This slimy layer coats not just the tank walls but also the substrate, decorations, and plants. If it coats your aquatic plants, they won’t be able to photosynthesize properly and may weaken and die.

There are many types of brown algae but what many fish owners refer to as brown algae isn’t algae. They are instead diatoms or single-celled algae that have walls made of silica. Silica is present in natural waters as it’s found in many forms of rocks such as quartz and sandstone.

Here are some steps tips for removing brown algae.

Give it time. Brown algae is a natural occurrence when setting up a new fish tank as the tank’s system needs time to mature. It takes about four to six weeks to cycle a tank, that is, to establish its bacteria and nitrogen cycle. Also, plants in new tanks don’t grow as rapidly and absorb as much nutrients as those in more established tanks. 

The brown algae should clear up on its own within a few weeks. At first, there will be few nutrients in brown algae, but it becomes more nutritious as it develops over time and fish will start to eat it. If it doesn’t clear up or if your tank isn’t a new setup, you may have other issues on hand.

Clean it. The best way to get rid of brown algae is with manual cleaning. Remove your jewelry and watch, wash your hands, and roll up your sleeves. Use scrapers, sponges, and magnetic scrubbers to remove the algae from the tank walls. Gently wipe the leaves of aquatic plants. Remove any decorations from the tank before cleaning them.

Bleach. If your tank decorations are coated with brown algae, soaking them in a bleach solution every few weeks will help. They’ll need to be rinsed thoroughly with water or soaked in water for a few hours to neutralize the bleach before putting them back in the tank.

Algae eaters. Otocinclus catfish, amano shrimp, and nerite snails are some of the sea creatures that will eat brown algae and some other types of algae. However, don’t introduce them to your new tank too early as they may start eating your plants. Add them one to two weeks after setting up your tank, when the brown algae gets more obvious. 

These algae eaters can have their own difficulties. Otocinclus catfish needs water to be well aerated and clean. Amano shrimp will eat the young shoots of plants when they run out of algae to eat, so it’s important to adjust the number of shrimp to the amount of brown algae in your tank.

All fish tanks will have some kind of algae, and it’s difficult to fully eliminate it completely. However, there are some ways in which you can limit the amount of algae growth. 

Test your water. If your tank has been recently set up, it will need more frequent testing of the water until a healthy environment has been established. Temperature, pH, nitrates, ammonia, and more can affect the quality of your tank’s water. For a more mature tank, regular water testing will help you maintain a healthy system. 

Change the water. Regular changes of water can help prevent algae from growing. This reduces the amount of nutrients in the water. Don’t change the water too often, though, as this can upset the balance of good bacteria in the tank. Also, don’t change more than 50% of the water at time to prevent stressing your fish.

Add more plants. Aquatic plants absorb the nitrates in water but just one or two plants won’t do much in your tank. At least 25% of your system needs to be plants for it to make a difference in the water quality.

Check the filtration. Many owners underestimate their tank’s filtration requirements. Pick a filtration system that’s about one and a half to two times the size of your fish tank.

Don’t overfeed. Any food that’s not eaten by your fish will rot and release more nutrients into the water which feeds the brown algae.

Activated carbon. Adding more activated carbon to the water filter can help absorb extra nutrients.

Use reverse-osmosis water. Regular tap water has phosphates and nitrates and if these levels are high, it can increase algae growth. Using reverse-osmosis or distilled water can help.