How to Control Aquarium Algae

Causes and Cures for Filmy Residue on Tank Glass, Rocks

Aquarium algae

Illustration: © The Spruce, 2018 

Algae growth is a fact of life that every aquarium owner will face sooner or later. Some algae growth is normal and healthy, but excess algae growth is unsightly and can be hazardous to fish and plants.

Algae may appear as a brownish, greenish, reddish residue or film that creeps up on tank glass, the gravel or substrate, accessories, and plants inside the aquarium. There are ways to avoid this overgrowth and, in many cases, reverse it, too.

What Causes Algae Overgrowth?

Like any plant life, algae thrive on three basic necessities: water, light, and nutrients. If there is an excess of any of these variables, then algae can grow like wildfire, just like weeds growing in a garden.

Obviously, your aquarium cannot go without water, but you can control the amount of light and nutrients that are in the water. Some common reasons for algae overgrowth are:

  • Lights left on too long
  • Aquarium in a location with direct sunlight
  • Overfeeding the fish
  • Going too long between water changes
  • Maintaining water environment with a high nutrient level

Avoiding Algae Overgrowth

Knowing what causes algae overgrowth is the first half of the battle. Next, work to prevent or rectify an algae overgrowth situation.

  • Reduce lighting: Do not place the tank where there is direct sunlight for even part of the day. Sunlight can, and will, promote algae growth. When using artificial light, make sure it is not stronger than necessary and is not on more than about eight to ten hours each day. Use a timer to turn the lights on and off each day.
  • Feed less: The majority of owners overfeed their fish, which increases the phosphate levels in the water. Feed small portions and watch the fish eat. If all the food is not eaten within five minutes, you are feeding too much. Always remove any uneaten food promptly.
  • Water changes: The single most important way to avoid algae is to perform regular water changes. Change 10 to 15 percent of your aquarium water every week to keep nutrients in the water low. This will remove the nitrate that accumulates in aquariums, one of the main fertilizers for plants!
  • Know your water: Test your water source. If it is high in phosphate, you should consider using phosphate removing chemicals available at your aquarium store, or find another water source, such as filtered water. Also, it is wise to test for nitrate, as some water sources have elevated nitrate levels. It does not do much good to change the water if you are adding nutrients back in the aquarium with the tap water!
  • Clean it up: If you see algae beginning to grow on the glass, rocks, or other hard surfaces of the tank, remove it. Scrape the glass, remove rocks, and scrub them. Vacuum the gravel when you perform water changes.
  • Keep live plants: Live plants will absorb many of the nutrients that algae thrive upon. Fewer nutrients in the water mean there is less fuel for algae overgrowth.
  • Keep algae-eating fish: Keeping Siamese flying fox, otocinclus, plecostomus, or other algae eating fish will help reduce some of the algae in the tank.

Types of Algae

If you notice an algae build-up in your aquarium, then you need to get rid of it. To do that effectively, you need to know the type of algae you have. Knowing the algae will help determine the cause and cure. Take a look at the common algae types, and how to deal with them.

  • Brown algae: Also known as gravel or silica algae, this algae is common in new tanks, and will coat the tank in sheets, which are easily wiped off. It is usually harmless and will eventually go away as the tank matures.
  • Blue-green algae: Also known as slime or smear algae, it can be caused by an excess of nitrate and phosphate in the water. In most cases, this is actually not algae at all, but cyanobacteria. It can spread rapidly and can be difficult to control. Good water care will help, but if your water source has phosphate in it, you may have to use special treatments to remove the excess nutrients. Erythromycin is effective against blue-green algae, but use it judiciously, it will likely harm the beneficial bacteria colony in your biofilter, too.
  • Red or beard algae: This is the toughest algae to get rid of, and it usually appears on plants. A dip in a weak (5 to 10 percent) bleach solution for a few minutes will often kill this type of algae.
  • Green algae: This is also known as hair, thread, or spot algae. It is a healthy type of algae that every tank will most likely experience to some degree. As long as the tank is well cared for, it will not overgrow. This is the algae that is eaten by algae-eating fish, so try using some of them in your aquarium for control.
  • Green water: Also known as an algae bloom, this is caused by the growth of microscopic algae that are suspended in the water. It is one of the more frustrating types of algae to remove as it cannot be wiped or scraped off like other algae. Generally, water changes are not effective, as the remaining algae will quickly grow back. Use of a diatomic filter or completely blocking all light for several days is usually necessary to conquer green water. Test the water to be sure it does not contain any ammonia, nitrite, nitrate or phosphate, as these will all promote algae growth.