How to Remove Algae from a Fish Tank

Medically Reviewed by Vanesa Farmer, DVM on July 13, 2023
4 min read

Algae are inevitable when you own a fish tank. With proper planning and preparation, you can ensure algae in a fish tank are kept under control, so this plant doesn’t become an issue for your fish. 

If you see algae in a fish tank, it’s important to identify what type of algae is present before attempting to treat it. Some algae are harmless, while other algae are dangerous and difficult to remove.

Brown algae. Sometimes it's called gravel or silica algae. Even though it's not pretty to look at, brown algae are harmless and very easy to remove. It's most prevalent in new tanks and may disappear with time.

Blue-green algae: This is also called slime or smear algae. It's caused by too much nitrate and phosphate in your tank water and is considered cyanobacteria. It grows rapidly and is difficult to control once it begins. 

Red or beard algae: The most difficult algae to get rid of, these algae usually grow on plants. 

Green algae: Some people call this growth hair, thread, or spot algae. It is good algae that every tank will experience at some point. As long as you take care of your tank, it will not overgrow. 

Green water: Also known as an algae bloom, it turns your water the color green because it suspends in water instead of growing on surfaces. Since you can’t wipe or scrape it off, these algae are very difficult to remove from your tank. 

Algae are plants and thrive in environments rich in water, light, and nutrients. This makes aquariums the perfect breeding ground. Too much light or too many nutrients in the water will cause algae to grow rapidly.

If you experience algae overgrowth, it could be caused by:

  • Leaving house lights on too long
  • The tank receiving too much direct sunlight
  • Too much fish food
  • Not changing water regularly
  • Allowing too many nutrients to build up in the water

You wipe and scrape all you want, but some algae will persist despite your efforts. Consider these tips on how to get rid of algae in fish tank water:

Take care of your tank's water. Be sure to check the water at least weekly to measure the levels of various things with special attention to pH levels. If you see a trend in them rising, invest in special treatments that will reduce excess nutrients from the water. 

You can use erythromycin for the growth of blue-green algae, but follow the instructions for use carefully because it can harm good bacteria growing in your aquarium, too.

Clean your plants. If algae grows on the leaves and stems of your aquarium plants, create a routine of cleaning them regularly. Using a solution of 5-10% bleach, dip the plants for a few minutes as needed to destroy the algae. Make sure they are thoroughly rinsed because bleach can kill your fish.

Invest in a filter. Remember, if algae persist through regular water changes, you have to do more to combat the problem. Invest in a diatomic filter that helps remove algae from the water consistently. 

You can try several methods to prevent algae in a fish tank.

Consider placement. Put your tank somewhere in your home with less direct light. If possible, keep your tank away from windows or ensure you have thick curtains to reduce the amount of sunlight coming in. Don’t leave lights on in the room for more than 8-10 hours per day.

Watch excess food. The general rule is that if your fish hasn’t eaten his food within 3-5 minutes, you’ve given too much. Cut back on how much food you put in the tank to prevent algae growth.

Change the water. While you shouldn’t change the tank water all at once, changing 10%-15% of the water each week ensures that nutrient levels are balanced. Over time, nitrate and phosphate accumulates and can contribute to algae growth. 

Test your water. Knowing about the pH, nutrient, and mineral levels of your fish’s water will help you know what changes to make. Chemicals are available at pet stores to help reduce the levels of some additives in your aquarium water. 

Try adding live plants. Live plants need the same nutrients as algae, meaning there is less for the algae to absorb for growth. 

Consider algae-eating fish. Fish like siamese flying fox, otocinclus, or plecostomus survive on algae. Before making a purchase, ensure they are compatible with the breeds already present in your tank.