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Why Is My Aquarium Water Turning Green?

Do you have green water in your aquarium? Learn how to get crystal clear water before a lethal mistake causes all your fish to die.

What is green aquarium water?

If the water in your aquarium is visibly green, it is due to an abundance of algae. Algae is an informal term for a large and diverse species called photosynthetic eukaryotic organisms with nearly 80,000 variations. These species primarily derive energy from two sources, the photosynthesis of light and excess nutrients.

Under optimal aquarium conditions, most aquarists pray never to see a bloom of these microscopic organisms. Of course, they’re always present in every aquarium, and that’s perfectly normal and healthy. However, when they bloom and get out of control, they will discolor your aquarium, turn the water green, and it may even become murky and opaque. With a few tweaks, algae can be easily managed to maintain a beautiful aquarium with crystal clear water.

What are the causes of green aquarium water?

The primary causes of green water are excess light and nutrients. Let’s look at how light affects aquariums first, then dive into how nutrients play a role.

Excess Light

Aquariums that are especially prone to algae blooms are those that receive direct sunlight. More often than not, excess light is the primary reason for algae in aquariums. This is because, like plants, algae synthesize light, and how sunlight helps plants grow, so does it help algae grow. Aquariums generally have lights to illuminate the tank for viewing pleasure unless it has living aquatic plants. Fish can live in the dark perfectly fine, which will be one of our methods for combatting runaway algae blooms.

Optimal light exposure for aquariums is 6-9 hours per day to minimize the risk of an algae bloom. Ideally, aquarium lighting is set for when the owner is home to enjoy the fish. It is perfectly fine to have your aquarium lights off during the day from 9 a.m. to 5 p.m. and then have them on from, say, 5 p.m. to midnight to 1 a.m. One of the best ways to control lighting is to remove the tank from direct sunlight and buy an automatic timer that controls the tank’s artificial light.

Excess Nutrients

The second most common cause of algae blooms is excess nutrients. If you have solid light control with optimal timing, but you’re still getting algae blooms and green water, then excess nutrients are likely the cause. The water that your fish swim in can be considered nutrient dense. These nutrients can easily become excessive because of too many fish and overfeeding. When fish overpopulate their ecosystem, their waste acts as a potent algae fertilizer. A common but much-disputed rule of thumb is that for every gallon of water your tank holds (minus the materials inside), you can safely hold one inch of fish. 10-gallon aquarium? Subtract about 1 gallon of space for substrate, plants, and decorations, so you have 9 gallons of livable water which can safely hold about 9 inches of fish.

Additionally, because some fish don’t understand the concept of being full, they will continue to eat if food is available, increasing the tank’s abundant waste. The fish that can feel full will allow excess food to drift to the bottom and become waste. Algae will gobble up all this uneaten food and begin to bloom. These effects are compounded if your aquarium has no live plants. Live aquatic plants help absorb the water’s nutrients and are one of the ways we can combat algae blooms from excess nutrients. How much should you feed your fish? First, know that fish can easily go for days without eating and be perfectly healthy. However, a good rule of thumb is once a day and only an amount that the fish can finish within five minutes. Fish generally take about 16-24 hours to fully digest their food, so multiple daily feedings are excessive.

What are the effects of green aquarium water on fish?

Green water is not toxic to your fish unless it becomes so abundant that it reduces the available oxygen for your fish. One of the main problems with algae is that it competes with your fish for oxygen. If you see your fish gulping for air at the water’s surface, keep in mind that this is a survival mechanism. The fish are signaling that they are in extreme danger. You must act quickly to save your fish. If the fish are swimming near the surface but not gulping, this is still a sign that you have an oxygen deficiency. This deficiency may lead to your fish gulping for air soon if the water quality further deteriorates.

What are the steps to get rid of green aquarium water?

Yes, you can get rid of that green water and have crystal clear water again. It can look as good as it did the day you bought it, and now that you understand the primary causes of green water, you are significantly better equipped to make this a reality.

Excess Light Solutions

  • Block out all sunlight for 3-5 days. Has the condition improved, but the algae still remains a problem? Continue blocking out all light and reassess after 3-5 days. Continue if necessary and begin assessing excess nutrient solutions.

  • Move the aquarium out of direct sunlight.

  • Use artificial light for 6-9 hours per day. The easiest method to control light intake is to use a timer on the light.

Excess Nutrient Solutions

  • One feeding per day that the fish can consume within five minutes.

  • Increase the water flow or current within your aquarium. Algae thrive in still water, so increasing the current will discourage their bloom. An easy way to do this is to buy an aquarium air pump or powerhead pump that helps circulate water.

  • Regular 15-30% water changes – usually once a week or once every two weeks. As you learn about your tank and the waste your fish create, you will become more comfortable with how long it can go without a water change. A well-maintained aquarium can go a month or longer without a water change.

  • Removing excess fish if you’ve surpassed the recommended limit of one inch of fish per one gallon of water as described above.

  • Adding live aquatic plants. They will help absorb excess water nutrients such as phosphates, ammonia, and nitrates. These plants will directly compete with algae, often starving the algae out and preventing its growth.

Additional Solutions

  • A UV-enabled filter that will help kill algae as water passes through. Depending on the size of the tank, this can be expensive and is not necessary with proper maintenance.

  • Introducing algae-eating fish, snails, or shrimp to the tank. This may help your problem, but it should not be the first resort, as increasing the bio-load to the tank by adding additional fish, snails, or shrimp may only worsen your problem. Even if these creatures eat the algae, they still create waste which, as described above, will act as a potent fertilizer for algae in the tank.

  • Introducing daphnia to the tank. Again, this last-ditch method isn’t sustainable in the long run because the daphnia will eat the algae and then be eaten by the other fish. It is a short-lived victory, however.

  • There are also chemicals sold by many manufacturers that can be tempting to buy as a magic solution, but they rarely work in the long run. It’s more of a band-aid that may help stop the bleeding. This includes special chemical filters.

Summary of algae solutions and prevention.

Excess Light

  • Block out all light to stop current algae growth.

  • Avoid direct sunlight.

  • Minimize artificial light 6-9 hours per day.

Excess Nutrients

  • Avoid overfeeding.

  • Increase the tank’s water flow.

  • Perform regular water changes and maintenance.

  • Don’t overstock the aquarium.

  • Add live plants.

Additional (last ditch efforts)

  • UV-enabled filter.

  • Add algae-eating fish, snails, or shrimp.

  • Add daphnia to the tank.

  • Safe chemical treatments.

Check out our other articles on algae!

Now that you’ve controlled the nasty runaway algae let’s talk about cleaning off those hard-to-reach decorations in your tank. Check out this article for a quick and easy bleach treatment that will safely and effectively neutralize any pesky lingering algae.

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