Watering Houseplants With Fish Tank Water

Watering houseplants with your fish tank water is a safe and sustainable way to conserve water while enjoying your hobby!

Watering Houseplants With Fish Tank Water

As climate change continues to alter rainfall patterns around the world, fish keepers are looking for ways to make the hobby more sustainable. Frequent water changes are a must for a healthy aquarium, but instead of discarding that dirty water you could use it for watering houseplants. Fish tank water is full of nutrients, so let’s talk about how you can conserve and reuse this precious resource!

How to Have a More Sustainable Home Aquarium

Hobby fish keepers face some tough choices when it comes to water conservation. While a robust filtration system can reduce the number of water changes fish need to stay healthy, there’s no way to entirely eliminate them. Fish tanks are closed systems, and the toxins that hurt fish build up in the water if it’s not swapped out periodically.

Depending on the size of your tank and how many fish you’re keeping, you may end up discarding as much as 20 to 50 gallons of water a month during your water changes! Even if you don’t live in a region where water is a scarce commodity, that’s a huge amount of waste. Why not reuse that water for your houseplants instead?

What’s in Your Aquarium Water?

The toxins that build up in your freshwater fish tank come from several sources. Leftover fish food and dying plant materials decompose and add to the waste in your water. The fish and other aquatic animals also produce waste that is broken down by your good aquarium bacteria into toxins like ammonia, phosphorus and nitrogen.

While toxic to your fish, these nutrients are very valuable to plants, both aquatic and terrestrial. Fish keepers often incorporate underwater plants into their aquariums which can make use of these nutrients. As long as you have a great light for your tank to support their growth, aquatic plants are one way to naturally clean and reduce the nutrients in the water. How can you use these nutrients to feed your land plants?

Methods for Reusing Your Fish Tank Water

There’s two popular methods for collecting and reusing your dirty fish tank water:

Directly Collect and Use Water From Your Tank

The first method is the easiest and doesn’t take any special set-up or equipment.

  • When it’s time for a water change, use a hose/siphon or gravel vacuum to collect the dirty water from your tank in a bucket or large bowl.
  • As long as the nutrients are not too concentrated, you can directly water your plants from the bucket with a cup or ladle.
  • If the nutrient levels are high because you haven’t changed the water recently, or if you’re using the water for a delicate plant or seedlings, you can dilute the dirty water with fresh water to prevent any harm to your plants.

The advantage of this first method is in it’s ease. Once you have your water change and plant watering schedules in sync, you’ll be able to use up the entire batch of dirty water on your houseplants, seedlings and flowers. You can even use the nutrient-laden water for your outdoor flowers, bushes and trees!

Set Up a Mini Aquaponics Systems

Alternatively, you could set up a small home aquaponics system and grow houseplants right on top of your aquarium.

  • Water continuously cycles from your tank up through the plant’s roots, and then back into your tank.
  • The roots take up the waste and nutrients in the water and use them to support the plant’s growth, cleaning the water naturally.
  • You’ll still have to do periodic water changes to replace the micronutrients in the aquarium water, such as calcium and magnesium, but not as often as in a normal fish tank.

The advantage of this system is it can be fully automated, and it’s not hard or expensive to design your own DIY aquaponics aquarium. It does take a bit more planning and effort than using a bucket to directly water your houseplants, but it’s the more sustainable option in the long run.

Is Aquarium Water Safe to Use on Plants?

I’m often asked on my aquarium website whether discarded fish tank water is safe for houseplants? As long as the nutrient levels are not too high, the water won’t burn your plant’s roots and you can always dilute the dirty water with fresh, if needed.

But that doesn’t mean it’s safe to use fish tank water on herbs or plants that you plan on eating. Aquarium fish may carry zoonotic diseases and parasites that can pass to humans in the water, such as salmonella. I don’t recommend using dirty aquarium water in your vegetable garden or orchard, or to grow edible flowers or seedlings.

Otherwise, watering houseplants with your fish tank water is a safe and sustainable way to conserve water while enjoying your hobby!

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