Why is my Aquarium Water Green?

Most of us in this hobby have at least witnessed an aquarium with green water. I have seen it plenty and even experienced it a fair amount in my aquariums. It sometimes occurs out of nowhere or from a water change. The green water you see plaguing your aquarium is a population explosion of critters in your water column. Those little animals are called phytoplankton. In most cases, green aquarium water is not dangerous to any aquarium inhabitants, it just looks awful. This article will discuss many ways to remove green water from your aquarium.

Before trying to make too many drastic changes to your aquarium, take a step back and give your aquarium a few days to “breathe”. Consider cutting down on feeding, cutting down or stopping fertilization, and taking a few days without water changes. The green water may go away on its own if you just let your fish tank work itself out. If this doesn’t work, the following are things you can try to alleviate your green aquarium water.

The first thing I would recommend is to evaluate how much light is getting into your aquarium. Throughout the day, check to see if there is ambient light entering your aquarium through a window. Next, look to see if someone keeps a light on in the room at all hours of the day. Lastly, reflect on how long you keep your aquarium’s lights on. Most planted freshwater aquariums only need light between 6 and 8 hours a day. I know I am terrible at tracking when lights need to go on and off in my aquariums. I bought a simple and cheap BN-LINK Indoor Timer (affiliate link) to plug my light into, so I don’t have to think about it. It saves me the headache of keeping my lights on too long if I get busy or will be away from home and miss the 6 to 8-hour photoperiod schedule. If you’re more tech-savvy than me the Govee Smart Plug (affiliate link) or any other smart plug may also be a better option for you.

If your aquarium is near a window or your fish tank gets a lot of indirect sunlight from a window, I highly recommend either purchasing blackout curtains or moving your fish tank to a darker area. If the light source getting into your fish tank is not removed, you’ll likely continue to get green pea soup water.

Water changes, liquid fertilizers, and too much feeding can introduce a large influx of nutrients into the water. This can be the spark that causes a population explosion of phytoplankton. Think about what you may have done recently to your water that could be the root of this problem. At the minimum, I recommend having a nitrate, ammonia, and phosphate test kit handy to keep an eye on the parameters that may closely affect algae and phytoplankton growth. The API Freshwater Master Test Kit (affiliate link) contains tests for nitrates, nitrites, ammonia, pH, and high-range pH. The API Phosphate Test Kit (affiliate link) will need to be purchased separately. If your test results have elevated levels of nitrates and phosphates, it may be time to conduct some water changes.

I would suggest at the minimum checking your tap water and your aquarium water for things like ammonia, nitrates, and phosphates. Algae and phytoplankton use these nutrients (among others) to grow. Also, the problem will not go away unless you physically remove the nutrients from the fish tank or have an abundance of plants that consume the nutrients. Phytoplankton are fantastic food sources for many different animals in the wild but are incredibly irritating in a fish tank. If you’re interested you can learn more about “What are Phytoplankton?” by checking out the EarthObservatory website.

Besides changing your light or your light schedule and reducing the amount of nutrients in the aquarium, there are a few other things you can do to combat the plague such as pea soup water. If possible, I would recommend adding all sorts of plants to remove nutrients from the water column. Floating plants are great at creating a barrier between light and water and they do a fantastic job at removing nutrients from the fish tank. Crypts, Vals, and Java Ferns are a few other species of plants that are easy to keep.

If you test your water parameters, and everything seems normal, you can see if there is an Aquarium UV Sterilizer that can help eradicate the nuisance.

When combatting things like algae outbreaks, I try to avoid using chemicals if possible. Many chemicals only band-aid the problem. What we have to do is remove the problem at its source. If you can figure out the source, whether it’s lights, nutrients, or something else, you can isolate it and make changes to your aquarium to remove it. If you’re also dealing with hair algae in your fish tank, consider checking out 8 Great Ways to Help Remove Hair Algae in Aquariums.