How to Deal With Algae on Gravel in an Aquarium

Algae on Substrate
Algae on Substrate Dan Isaacs

No matter what you do, if you mix fish with water and light, you will get algae. Most of the time, minimal algae will not hurt your fish. It is mostly an eyesore that most humans can't stand. Heavy algae blooms can compromise your fish's oxygen consumption and tank pH, though, so some maintenance will be required. There are many different types of algae than can colonize your tank, including algae that grows on gravel and other substrate.


Although most aquarium algae is benign, be on the watch for cyanobacteria or blue-green algae. This algae can contain a serious respiratory and neurologic toxin for humans, other pets, and your aquatic pets.

Signs of Algae on Gravel

Algae can grow on many surfaces of your tank, including your decor, plants, and substrate, including gravel. You can also have various colors of algae, including red, brown, green, or gray. Algae can grow in flat patches or raised bunches. Most commonly, flat, brown algae collects on gravel. It may cause a slight discoloration or a dense mat.

Causes of Algae on Gravel

The most common cause of algae on aquarium gravel is a lack of proper maintenance. All aquariums should use a gravel vacuum to remove debris and detritus from the substrate on a regular basis. By using a gravel vacuum, you turn over the gravel substrate, depriving it of light and killing the algae.

Extended light periods can affect algae levels. If you do not have corals or other light-dependent organisms in your tank, shortening the light/dark cycle can help decrease algae levels.

Excess algae in aquariums is also secondary to high nitrate or phosphate levels. Nitrate, the end of your nitrogen cycle, is used by plants and algae as a food source. If you have been lax in your maintenance protocols, your nitrate levels will increase and provide more food for algae to flourish. Phosphates are more commonly associated with high source water levels. These may be difficult to remove and may require finding a new source water.

How to Fix Algae on Gravel

The best method to fixing algae on aquarium gravel is to cut off its food sources: light, nitrate, and phosphate. This is mainly done by performing regular maintenance using a gravel siphon. Vacuuming will turn over your substrate, cutting off its access to light, and remove nitrates and phosphates from your system.

All fish species have a different tolerance to high nitrate levels. Some fish can tolerate very high levels, whereas others cannot tolerate much at all. Do your research prior to bringing your fish home so you know what water chemistry parameters you have to maintain. Test your water chemistry regularly to make sure that all your levels, including your nitrate, are within range.

Phosphates can be more difficult to remove. In freshwater aquariums, they are often high in your source water. In saltwater aquariums, they may be added to various salt mixtures since some coral species require phosphate. If you do not have any corals in your tank, you do not need to use a coral-specific mix. Phosphate-binding filter media can be added, but consistent problems with source water levels may require using a different source.

Algae Prevention

Unfortunately, there is no guaranteed method to avoid algae altogether. The best method to prevent a heavy algae build up is to keep up with your regular tank maintenance. Your maintenance routine will vary based on the tank species, the number of animals and their diet. Use a gravel siphon to turn over your substrate, blocking light access and removing excess food and waste. Test your water chemistry regularly to make sure your nitrate and phosphate levels are not making the algae issue worse.

UV lights only kill algae cells in suspension and have no effect on algae already established in your tank. A UV light may help keep the problem from getting worse, but it will not kill any algae attached to your substrate. UV lights do not treat any parasites or bacteria affecting your fish.

Remember, in low amounts, algae is more of a nuisance to humans; your fish couldn't care less. A tank overrun with algae, however, can have serious health consequences for your fish, affecting oxygen and pH levels between the daylight hours. Keeping up with your maintenance routine is the best thing you can do to keep your fish healthy and your algae levels in check.