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Green Water -- Causes and Cures

Has this ever happened to you? Your aquarium is running smoothly, the water is crystal clear, the fish are swimming happily about, everything is great! Suddenly, out of nowhere, you wake up one morning to find your beautiful showpiece has turned to... PEA SOUP!! The water is electric green and you can barely see into the tank. You frantically do a big water change and put a new cartridge in your filter. Things improve for a day or two, but then it's back. This time with a vengeance! What's going on and how do you fix it?

"Green Water" outbreaks are caused by a sudden population explosion of suspended algae known as phytoplankton. Unlike other algae species that grow on the glass or objects in the aquarium, green water algae float about the tank and multiply by the billions in a short period of time, in what's known as a "bloom". They're typically not harmful to your fish and other aquarium critters, or you, for that matter, but they are unsightly and severe cases can block light to plants. Interestingly, some phytoplankton are beneficial. Many saltwater reef enthusiasts grow or purchase phytoplankton cultures specifically to feed to their corals, and freshwater breeders cultivate phytoplankton called infusoria to feed the tiny fry of certain species. But for most aquarists these algae are something to be discouraged or eliminated. Quickly.

What Causes Green Water Outbreaks?

Excessive Light

Algae are plants, and plants love light. However, excessive light in the presence of a source of nutrients (we discuss this below) can send algae into hyperdrive. Too much light can come from placing your aquarium in a sunny window, leaving the tank light on too long, or using a light that is too strong for the aquarium.

Nutrient Imbalance

Plants need nutrients to grow. For most algae that means nitrate and phosphate, which typically come from fish food and fish waste, but they can be in tap water as well. Overfeeding and/or having too many fish for your tank size or filter capacity also lead to a build-up of nutrients. Performing water changes with nutrient-laden tap water will have the same effect.

Poor Maintenance Habits

Lack of water changes and proper maintenance causes a gradual deterioration of water quality. Over time this creates an ideal environment for algae to grow. Since we can't see nutrients, we have no idea how bad conditions are until something bad happens, like our fish getting sick or an algae bloom occurring.

How To Prevent Green Water Outbreaks

Proper Lighting

Avoid placing your aquarium in a sunny window. If you can't avoid doing so, install background material or draw the shades during the daytime to eliminate as much sunlight from hitting your tank as possible. If your aquarium doesn't receive direct light but is in a bright room, keep the tank light off during the day.

Choose a light suitable for the size and water depth of your aquarium. If you don't have live plants, you don't need a super-powerful light. Install a timer to control the number of hours the light is on and provide a consistent day/night cycle. Planted aquariums need 8 to 12 hours of light daily, while non-planted aquariums can get by on 6 hours or less.

Good Maintenance Habits

Regular water changes are essential to maintaining a healthy aquarium and preventing algae outbreaks. 10% weekly or 25% bi-weekly is a good place to start. Siphon out uneaten food and other organic material and lightly vacuum the gravel. (Avoid vacuuming gravel if you have a newly set up aquarium or a planted tank that uses substrate fertilizers.) Make sure replacement water is free of nitrate and phosphate.

Replace hang-on filter cartridges monthly, and service canister filters regularly. Rinse or replace mechanical filter media and change activated carbon or other chemical media every 4 to 6 weeks.

Water Testing

Test your aquarium water regularly for nitrate and phosphate and take steps to correct levels that are too high. Tap water can contain high levels of nitrate and phosphate also, so it's a good idea to test yours before using it in your aquarium. Use reverse osmosis (RO) or deionized (DI) water with Aqueon Water Renewal added if your tap water contains high nitrate or phosphate levels.

Proper Stocking & Feeding

Having too many fish in your tank or feeding them too much will cause nitrate and phosphate levels to rise, which algae love! Maintain 1 inch of adult fish per net gallon of water, and feed only what your fish can consume in 2 minutes or less, once or twice a day. Many veteran aquarists skip feeding at least one day per week.

Live Plants

Since aquatic plants use the same nutrients as algae that cause Green Water outbreaks, they are a very effective way of preventing them. In addition, floating plants like duckweed, hornwort and others help reduce the amount of light penetrating deeper into the water. Well-planted aquariums almost never experience algae problems.

Proper Dosing

When dosing liquid plant fertilizers err on the side of less vs. more, especially if you don't use CO₂ or don't have a lot of plants. Remember, algae are plants too and they'll make use of any excess supplements that your plants aren't taking in. Also, keep in mind that an aquarium never holds as much water as its designated size. Example: A 20 gallon aquarium typically holds about 16 gallons of actual water after taking into consideration gravel, rocks and other decorations. Dose accordingly.

How to Cure Green Water Outbreaks

UV Sterilizer

The most effective and effortless way to get rid of Green Water is to install an Ultra-violet Sterilizer on your aquarium. As water passes through the UV chamber, suspended algae are eliminated, along with many disease-causing organisms. The result is crystal clear water in a matter of days. Ultra-violet sterilizers are completely safe for fish, invertebrates and plants and they have become increasingly affordable and easy to install in recent years. With a UV sterilizer on your tank, you should never have a Green Water problem again. The Aqueon QuietFlow UV Ultraviolet Sterilizer is designed for aquariums up to 40 gallons, and Coralife Turbo-Twist UV Sterilizers are available in 3 sizes, rated for aquariums up to 500 gallons.

Eliminate Light

Excess light is often a trigger for Green Water algae blooms. One way to get rid of an outbreak is to temporarily eliminate light altogether. Turn the aquarium light off and wrap the tank with black plastic, trash can liners or blankets, and keep it covered for a few days. Uncover the aquarium long enough to feed your fish each day, then cover it up again. In many cases the algae bloom will die off within a few days, but you will still need to address the cause of the bloom. If you don't see results in 48 to 72 hours, another course of action is recommended.

Diatom Filter

Although they're considered old-fashioned and can be pricey, these filters use diatomaceous earth to trap microscopic particles – including suspended algae – creating crystal clear water. Some models can be fitted with regular-use media so they can be used for standard day-to-day filtration as well.

Chemical Treatment

There are many products out there that are effective at getting rid of Green Water algae blooms, but it's important to note that chemicals should always be a last resort. For one thing, they do not address the cause of the problem, and you can never be sure of their effects on fish, plants or the balance of your aquarium. That said, in many cases they do work. Aqueon Algae Remover is effective against many types of algae, including those that cause Green Water outbreaks. When using any chemical treatment, remove activated carbon and other chemical filtration media from your filter, maintain good water circulation in the aquarium, and dose according to net gallonage, taking into consideration water volume displaced by gravel, decorations and other objects in the tank.

By maintaining a healthy, balanced aquarium and understanding the causes of Green Water algae blooms and, they shouldn't be a problem in your tank!

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