Is Used Fish Tank Water Good for Plants?

Is Used Fish Tank Water Good for Plants?

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If you have an aquarium, chances are there’s extra water siphoned off after cleaning it. You have to dispose of it somewhere and, in some municipalities, it’s illegal to dump it down the drain or the sink. This is because it can contaminate the local water supply, especially if you have something like a Turtle.

So, if you also have indoor or outdoor plants, you may be wondering whether you can give the used water to them.

Used aquarium water contains is high in ammonia and other chemicals often very harmful to fish. But, if you have plants around your home, you can give the used water to them. It’s gardening gold because this water acts as an all-natural fertilizer to enrich soil and expedite plant growth.

Is used fish tank water good for plants?

It may seem a little gross in concept to give your plants water that contains fish poo and uneaten food. But, used fish tank water is excellent for plants. It’s nutrient-rich, organic and filled with ammonia, nitrogen, phosphorous, potassium, iron and other trace minerals. These are all the same things you find in commercial fertilizers.

This is the best nutritional boost you can give your green-growing things all year round. What’s more, it’s already diluted so it won’t burn the plants, making them healthy and lush. Some gardeners report improvements of up to 200% because they use “dirty” water from their aquariums.

Are there any exceptions to giving plants used fish tank water?

There are a few exceptions to watering plants with used aquarium water. First, if you have a saltwater tank, DO NOT give used water to your plants because the salt will kill them. What’s more it will demineralize the soil, rendering it useless and infertile. This will be particular for potted plants.

If you’ve treated fish diseases along with added chemicals to kill algae or to adjust the pH level, do not give used aquarium water to food plants. When it comes time to eat the fruits and vegetables watered with such chemicals, it will make you and others very sick.

In fact, don’t give any used aquarium water to plants you intend to eat, just the ornamentals. This is because the higher concentrations of waste products in the soil will infuse into the food. Once consumed, a person can get very ill. But, if this is something you take interest in, consider having an Aquaponics set up, briefly discussed below.

Can you still give plants used tank water if you haven’t cleaned the tank in awhile?

In the event you neglected to clean your tank, you should dilute it with fresh water before watering your plants. This is because the water will be far too concentrated and has the potential to burn your plants, particularly the potted ones.

Is there a way to ensure used aquarium water will be appropriate for plants?

As long as you have a freshwater tank, there’s a way to ensure your future used water will be good for plants. Because you have to treat most tap water before using it in your tank, it means you have to put in some chemicals, conditioners and other additives to make it suitable for fish. This cocktail won’t be any good for plants.

There are two ways you can achieve this. First, you can try aging your water or you can shift your aquarium and gardening into one hobby, called Aquaponics.

Aging Water Ahead of Time

Aging your water before adding it to the tank will help relieve stress to your fish by controlling the water parameters. If you have any fish sensitive to even the slightest change in temperature or chemistry, aging it ahead of time will help keep them happy and prolong their lives.

There are two methods to age water. The first is to fill a clean bucket or jug with tap water and let it sit uncovered for seven to 10 days. If you use filtered tap water, you may be able to truncate that down from anywhere between five to seven days. This one is more time-consuming and is best for smaller quantities.

The other way you can age water is by filling an extra tank with tap water, which allows for a larger volume. This also means you can adjust water parameters to prime levels, like pH balance (so long as there aren’t any harsh chemicals), heat, filtration, aeration and more. What takes several days in a bucket can happen quick overnight.

Aquaponics Basics

Aquaponics is another option you can use to guarantee growing food plants while having a neat aquarium to enjoy at the same time. Aquaculture is the ability to grow plants and fish in the same recirculation of water. You won’t need much, if any, soil.

With a combination of heating, air pumps and water pumps, you’ll be able to raise fish while that water circulates to plants. The plants have bacteria that will convert the ammonia and nitrite into nitrate while absorbing proper amounts of phosphorous, potassium and other minerals.

This plant activity helps to filter the water (along with an additional filter) that goes back into the aquarium. And this makes it useful and healthy for the fish. The cycle continues while providing you with fresh fruits and vegetables to eat.

What should I do with dead fish in the water?

If you have a dead fish in your used tank water, bury it in your garden. The plants will love the decaying matter. Believe it or not this is an ancient practice that comes from Native American cultures, who added dead fish to the soil. They actually taught this to the first settlers of North America.

What should I do with used aquarium water if I can’t water plants with it?

Of course, the natural inclination is to dump it directly down the sink drain or the toilet. These will be your only options in the event you can’t water your plants with it. So, you have to neutralize the water and dilute it before you do this.

Simply mix in some fresh tap water and some baking soda until well dissolved.  Allow this mixture to rest about 20 to 30 minutes. Then pour it down the drain or flush it down the toilet.


Used aquarium water is a literal gold mine for plants. So long as it comes from a freshwater tank free of harsh chemicals, your plants will thank you. It will make their foliage and flowers beautiful, green and full. But, you don’t want to use this with food plants unless you’re creating an Aquaponics system in your home.

Applying used aquarium water is a good option, whether using Aquaponics or recycling the water to your plants.  This is especially true if you live in an area where dumping aquarium water is illegal. But, if you always have to treat your tap water before putting it into the tank, it may be better to age it before hand.

But, giving your garden or potted plants used aquarium water is a wonderful self-sustaining system. What eventually becomes poisonous and toxic to your fish will be fantastic food for your green-growing things. It may seem a little unsavory in concept, but it’s the chemistry going on creates the benefit.

See Also:
How Do You Know if Your Algae Eater Is Dying?
How to Clean Fish Tank Gravel Without a Vacuum
Can You Use Rainwater In Fish Tank?
Can Fish Tanks be in Direct Sunlight?