Is Fish Tank Water Good for Plants?

Sarah Metzker Erdemir

Regular water changes are necessary for keeping a fish tank healthy. Fish feces, uneaten food and deteriorating plant matter all build up in the tank, which makes the water look dirty and is potentially toxic to the fish if left for too long.

Aquarium keepers dump out water anywhere from once a week to once a month, depending on the size of the tank and the number of fish, but there is a greener way to dispose of this water.

What's In Fish Tank Water?

Before you pour your aquarium water down the sink, consider using it to water your plants. This not only saves water by recycling it, it saves you money on expensive fertilizers. Aquarium water is a rich source of nitrogen, which is what plants need to make chlorophyll, an essential component of photosynthesis. It also contains phosphorus, which supports healthy root growth, and potassium, which makes plants strong and helps them hold water. Used fish water also contains beneficial bacteria that break down ammonia and help feed the soil. Water from outdoor fish ponds can also be used to water your garden.

Organic Fish Fertilizers

Organic fertilizers such as fish emulsion and fish meal contain the same essential elements that support healthy plant growth. These fertilizers are highly concentrated and meant to feed plants once or twice a year, and they smell bad. If you have an outdoor fish pond, the sludge from the filters contains a lot of the same material as manufactured fish emulsion. Aquarium water is a diluted version of these fertilizers and can be used more frequently to hydrate and feed plants. It’s a little bit smelly, too, but the odor usually fades within an hour.

How To Water With Aquarium Water

Indoor plants and outdoor gardens will both benefit from fish tank water. Not all fish tank water is appropriate for plants, however. Salt water from marine tanks cannot be used as fertilizer because the salt will kill the plants. Additionally, chemical pH adjusters or fish medications used in some aquariums are not safe to use on fruit and vegetable plants that you plan to eat. If you have neglected to change your aquarium water for a long time and it is very dirty and bad smelling, use it sparingly on plants or you risk overfeeding them. Sludge from aquarium and outdoor pond filters can also be fed to plants, but use it as you would fish emulsion or dilute it with water first.