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Watering Plants with Aquarium Water: Reuse vital nutrients

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Fish tank water can be very beneficial for both house and garden plants. Changing the fish tank water is necessary to ensure the right environment is maintained in order to have happy fish. You can save and reuse the water on your plants.

Watering plants with aquarium water is beneficial because it contains nitrates, the form of Nitrogen that plants can absorb easily. Plants synthesize nitrates to produce green leafy branches, which boost photosynthesis and create the food necessary for healthy growth and development.

The water from the fish tank should not be thrown down the drain. You are actually throwing away fertilizer that you probably have to get for your plants. There is soo much more to fish tank water than what meets the eye.

Is Fish tank Water good for plants?

The fishes in a tank will carry out their normal day-to-day fishy chores and by doing so, over time, the fish tank becomes dirty and eventually needs to be cleaned.

Water changes are done to a fish tank to remove excess chemicals and other materials that could harm the fish if allowed to build up.

The water from the fish tank may not be beneficial to the fishes afterwards but are definitely beneficial to plants as it contains nitrogen in the form of nitrates.

Now let’s get a little scientific for a bit.

Plants need three main elements to maintain good growth.

Any plant owner will tell you its Nitrogen (N), Phosphorus (P), and Potassium (K). Plants require adequate amounts of NPK for optimal growth.

The water from the fish tank has mainly Nitrogen in the form of nitrates. Without nitrates, a plant cannot grow and produce flowers and seeds.

This is primarily because nitrogen is an essential component of plant proteins and chlorophyll, which is necessary for the process of photosynthesis.

Fish food also contains proteins that can be used as a fertilizer for plants. That’s another option for feeding your plants.

In the fish aquarium lives good bacteria that take the toxic ammonia from the fish poop and break it down into nitrites (not used by plants). It then further converts the Nitrites into Nitrates which the ingredient that plants love.

If you take a closer look at any fertilizer packaging you would see some part of the ingredient name as (weird chemical name)-Nitrogen.

eg. Ammoniacal Nitrogen

So this is what you are paying for when you purchase fertilizer at the plant shop. don’t throw it away because it is very beneficial to your plants.

If you want a slow-release fertilizer that will ensure your plant grows healthy for a long time with minimal intervention, I recommend Osmocote Smart-Release Plant Food.

Osmocoat fertilizer

Using Fish Tank Water is Similar to Aquaponics

The practice of using fish water to water plants have been around for thousands of years. This is why you find many early civilizations settling close to rivers and waterways.

Aquaponics is the combination of aquaculture (growing fishes in tanks) and hydroponics (growing plants in nutrient rich water), where microbes converts the waste from fishes to nutrients, for plants to grow, in a recirculating system.

The fish produces the fertilizer and you feed the fertilizer to the plants.

Instead of having an entire aquaponic system, you have your pet fish producing beneficial fertilizer for plants to grow.

For a more detailed explanation of aquaponics and how it works, I have done a comprehensive article explaining everything about aquaponics.

Types of Plants Fish Water is Good For

Water from the fish tank will be beneficial to all types of plants. But there are some plants that actually love it a lot more than others.

Green leafy plants, in general, need more nitrogen than flowering and fruiting vegetation. Here they are.

  • Lettuce
  • Pak choi
  • Bok Choi
  • Cabbage
  • Tomatoes
  • Cucumbers

When Not to Use Water from the Fish Tank

Although it is good to water plants with fish tank water. having too much poop in an aquarium may be bad not only for the fishes but also for the plants that are being watered.

Excess buildup of fish waste may cause excess ammonia to form. This happens because the bacteria in the tank cannot handle the amount of work it has to do. In turn, the water becomes dirty and mucky with poop.

Ammonia is toxic to both the fish and plants and should not be used to water plants.

Instead, it should just be thrown away. If you insist on using the water. Just keep it in a container for a couple of days, maybe a week. This would give the bacteria in the water enough time to do their thing.

Keep the water in a dark place because light would cause algae to flourish and use up the available nitrates from the water if it’s left where there is light.

After the time has passed it will be safe to water plants as the ammonia will be converted to useful Nitrates for the plants.

Don’t water plants if you have recently used any chemical additives such as pH up or pH down to regulate the water conditions in the fish tank. The additives will also affect the plant. So be mindful of this also.

Using Water from a Salt Water Fish Tank

In this case, you should avoid watering the plants with salt water at all costs. Instead of adding hydration, you will be dehydrating the plants instead.

Most plants can tolerate saltwater on their leaves and stems, but they will dehydrate if they drink salt water from the soil. Even if they don’t dehydrate, they may be poisoned by an excess of salt in their systems.

Plants need a small amount of salinity to survive, since salt is one of the nutrients necessary for plants to grow, so the presence of some salt is necessary.

However, saltwater has a high concentration of the mineral, which is why it can be poisonous to most plants.


Watering plants with fish tank water is beneficial to the plant since it contains nutrients that plants use to grow. If you are using chemicals to regulate the water then you should not use them.

However, water from a saltwater tank is a definite no, since the water contains salt that will cause more harm than good.

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