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Why is your aquarium water green?

Your aquarium may have started out with crystal-clear water, but over time it got slightly green, then darker and darker. Now, you’re looking through foggy green water and can barely see your fish. It’s a common problem, but there are many ways you can fix it.

If you’re finding yourself wondering, “Why is my fish tank green?” this information can clear up the mystery — and help you clear up your tank.

What turns aquarium water green

When you understand what turns aquarium water green, you can take the right steps to solve the problem. At the most basic, green aquarium water is caused by tiny bits of algae.  These living bits are called phytoplankton, and you can only see them with a microscope. When phytoplankton become too concentrated in your tank, they’ll turn the water green, also called an algae bloom. If left untreated, your aquarium water can turn a thick yellowish green.

In most cases, this greenish water isn’t harming your fish or aquarium plants, but it’s definitely ugly to look at.

There are three potential causes of too much phytoplankton.

Excessive light

Remember that phytoplankton is algae, a type of plant, and light helps drive plant growth. If you’re using an aquarium light that’s too strong for the tank’s size, or if you’re leaving the light on too many hours per day, you could be driving the algae growth.

Sunlight can also be the culprit here. If your tank is positioned near a window, even with the tank light off, it could still be overexposed to light, allowing the algae to keep growing.

Nutrients that are out of balance

Your aquarium plants need the right balance of nutrients to grow, but algae also feed on those nutrients. If you add nutrients to your tank, too many or too few could be skewing the balance, allowing algae to thrive.

Not enough tank maintenance

Your aquarium needs regular maintenance to keep that beautiful, clear water from getting cloudy and green. Basic maintenance like water changes, filter cleanings, and water quality testing is important to the tank’s environment. If you’re not doing this maintenance as often as you should be, you could be contributing to the algae overgrowth.

Mother and son looking at aquarium
Westend61/Getty Images

How to fix your green-water problem

To get rid of your green water, you’ll need to first identify what’s causing it. In some situations, multiple factors can contribute to the issue and determine possible solutions.

Use a UV sterilizer

If you want to get rid of your green water right away, an ultraviolet (UV) sterilizer can help. The UV chamber helps eliminate algae as water flows through it, so you could start to see clearer water in just a few days.

A UV sterilizer will solve your issue, but green water can indicate that something else is amiss in your tank. To identify just what’s off, you might want to take the following steps before installing a UV sterilizer.

Reduce light

If too much light might be prompting excessive algae growth, start by reducing the amount of light available to your tank. Use your tank light for only eight to 10 hours per day, and try swapping in a lower-wattage bulb. If your tank is by a window, move it somewhere that receives less natural light and see if that makes a difference.

Fix water imbalances

It’s also a good idea to check the levels of the chemicals in your water. Testing your water can reveal imbalances in nitrate and phosphate levels, which can prompt algae growth. If you have too many fish in your tank, they’ll drive up your nitrate and phosphate levels. Reducing your number of fish or putting them in a larger aquarium can help solve this water imbalance.

You can also add in more plants. Plants consume the same nutrients that algae need. More plants means fewer nutrients for the algae, reducing algal growth.

Clean your tank

Even with all the above steps, regular tank maintenance and cleaning are essential to prevent green water and algae. Change out at least 20% of your tank’s water every week, and set yourself a calendar reminder so that you don’t miss these important changes.

Plan regular deep cleanings, too. It’s possible to deep-clean your aquarium in just one hour, so add these cleanings to your schedule. Be sure to also put filter change due dates into your calendar. Regularly changing your tank’s filters will allow the filtration system to perform at its best. This can even lengthen the life of your filtration system.

Solving the algae issue

It may take some time before you identify just what’s causing your green-water issue, but following these steps can help clear up your water. With regular cleanings and correct water balances, you’ll be making your aquarium a healthier environment for all your creatures.

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