Why Is My Aquarium Water Green?

If your aquarium has turned a cloudy shade of green — sometimes called "pea soup" — you are dealing with a green water algae bloom. It is one of the most common and frustrating problems in the hobby, but the good news is that it is almost always fixable once you understand what is feeding it. This guide from Tropical Treasures Wyo in Cheyenne, Wyoming explains what green water actually is, what causes it, and how to clear it for good.

First, it helps to know that green water is different from a greenish film on the glass or fuzzy growth on plants. Those are surface algae. Green water is caused by free-floating, single-celled algae suspended throughout the water column, which is why it tints the entire tank rather than coating one surface. If your problem is on the glass or decorations instead, our guide to identifying every type of aquarium algae covers those. And if your water is tinted brown or amber rather than green, that is usually tannins — see why aquarium water turns brown.

What Causes Green Water?

Green water blooms come down to a simple imbalance: too much light and too many nutrients, with nothing to keep the microscopic algae in check. Any one of the following can tip a tank over the edge, and they often stack up together.

Too Much Light

Direct sunlight on the tank is the single most common trigger, especially for tanks near a window. Running the aquarium lights too long — more than eight or nine hours a day — has the same effect. Algae are plants, and excess light is fuel.

Excess Nutrients

Floating algae feed on nitrate and phosphate, which build up from overfeeding, overstocking, and infrequent water changes. A tank with high nutrient levels and bright light is the perfect storm for a bloom. Our guide on how to lower nitrates walks through bringing those numbers down, and the only way to know your levels is to test — our comparison of aquarium test kits can help you pick one.

A New or Disturbed Tank

Brand-new tanks that have not finished cycling, or established tanks that were recently deep-cleaned, often bloom because the system is briefly out of balance. If your tank is new, our nitrogen cycle guide explains why this happens and how to let the tank stabilize.

How to Clear Green Water

There is no single switch, but combining a few of these approaches reliably clears a bloom and keeps it from coming back.

Cut the Light

Move the tank out of direct sunlight or block the window, and reduce your light schedule to around six to eight hours a day on a timer. A complete "blackout" — covering the tank entirely for three to four days — is a classic and effective way to starve floating algae, and it does not harm most fish. Healthy plants can ride out a short blackout fine.

Reduce Nutrients

Feed less, remove any uneaten food, and increase the frequency of your water changes for a week or two to dilute the nutrients fueling the bloom. Going forward, a consistent routine prevents the nutrient buildup that starts blooms in the first place.

Add Competition and Filtration

Fast-growing live plants out-compete algae for the same nutrients and light, which is one of the most durable long-term fixes. If you want to lean into that, browse our live aquarium plants or start with our easy beginner plants. A strong, well-maintained filter helps too — our aquarium filtration guide covers keeping it running well. For a stubborn bloom, a UV sterilizer clears the water by killing the free-floating algae as it passes through the filter.

Is Green Water Harmful to Fish?

Green water itself is usually not dangerous to fish, and in fry-rearing tanks it is sometimes cultured on purpose as a food source. The bigger risk is indirect: a heavy bloom can swing oxygen levels overnight as the algae stop producing oxygen and start consuming it. If you notice fish gasping at the surface or gasping during a bloom, add aeration. Note that green water is different from a milky, white-gray haze, which is usually a bacterial bloom — our guide on cloudy aquarium water covers that separate issue.

Preventing Future Blooms

Keep lighting moderate and off direct sun, feed sparingly, test and manage nutrients, stay consistent with water changes, and keep live plants growing. A balanced tank rarely goes green. If a bloom keeps coming back despite your best efforts, bring a water sample to our shop in Cheyenne for free testing and we can help you find what is feeding it.