Is aquarium water good for plants?

Last Updated on November 8, 2021 by cmoarz

Have you ever wondered what you could use old fish tank water for instead of just pouring it down the sink? Does it have other uses? Can it be used to water plants including indoor plants?

Old aquarium water is densely packed with micronutrients, but is it too much? Will they be safe? We answer all these questions and more!

What’s in Fish Waste?

The big 3, Nitrogen, Phosphorus, and Potassium (NPK for short). It’s also chalked full of other macronutrients such as calcium and magnesium, as well as many micronutrients and other trace nutrients.

Dirty aquarium water from healthy fish tanks is a soup of beneficial bacteria and delicious vitamins, minerals.

While it might be a bit too strong for aquatic plants and fish, it makes a wonderful natural plant fertilizer for your garden plants, be it tropical plants, vegetable garden, and any manner of potted plant and other types of plants.

Is Fish Tank Water Good for Plants? Can It Harm Them in Any Way

There are some exceptions! In most cases the answer is no, fish tank water can’t harm plants. The exception being salt water, and medicated water.

Salt water is a kind of a no-brainer. Putting salt on vegetables is one thing, putting salt on vegetable plants is an entirely different beast.

The same goes for all other plants too. Most plants do not like salt. So when watering plants with used aquarium fish poop, I suggest avoiding doing so with saltwater tank water and sticking to freshwater aquariums only.

Another issue is with medicated tanks. Medication may be good for the fish but it’s not always going to be good for your plants. Excess chemicals, heavy metals, and other harmful chemicals make up many medications that are used in aquariums.

To remove excess chemicals will most likely take more money and time than it’s worth. And in removing the various chemical additives, you will most likely be stripping the water of anything worth giving to your plants, to begin with.

So it’s best to avoid watering plants with aquarium water if it’s been medicated in any way, that is if you want to keep healthy plants.

As for the tap water itself, Generally, it’s already been treated before being put into an aquarium. So chemicals like chloramine and chlorine aren’t something you need to worry too much about.

So with all that said, yes, fish tank water is absolutely amazing for plants and won’t harm them if you avoid medicated water and saltwater.

Another myth that needs busting is you should only use the fish tank water on ornamental plants because it’s not healthy for anything you intend to eat.

This isn’t usually true unless the water has been medicated or neglected for a very long time. There is a reason why fish water has been used to help plants grow in aquaponics for years.

In fact, Having fish live in a tub or container connected to the hydroponics provides more nutrients plants love as well as excess water availability for your plants.

This system of hydroponics has been around for quite a while now and is exactly what you would expect.

It saves money, provides a great symbiotic relationship, lowers the number of water changes needed in a hydroponics system, and gives almost everything plants require to grow and be happy.

So, only you know what’s happening in your own aquarium water, so you are the best judge of character for that water, rather than a static answer of safety provided by this article.

With that said, always wash and clean your fruit and vegetables, especially from houseplants that don’t have a lot of creepy crawlies to break down much of what you add.

How Do You Prepare and Use Old Fish Tank Water?

You need not go to any great lengths to use fishtank water. Just dump it on the soil like you would normally water any plant. The solution should be dilute enough to avoid any issues such as plant burn.

You can also add it to a spray bottle to get a nice mist going on the leaves!

There are of course other ways you can prepare and use old fish tank water that is a bit more involved, while simultaneously more beneficial while maintaining the all-natural appeal.

But it involves testing the water itself and adding appropriate fertilizer additives to make a sort of tea.

Speaking of tea, If you’ve got a worm bin, you can steep worm castings in the aquarium water to make nitrogen, nutrients, and beneficial bacteria dense castings and fish tea fertilizer for your garden plants that has nearly everything plants require in it.

And, to add too that, Don’t flush your dead fish down the toilet. Give them a proper burial in your garden. The ammonia, nutrients, and other materials will make great compost for your plants!

Can Fish Tank Water Replace Fertilizer Completely?

While excess old fish tank water good for plants, it doesn’t completely beat out traditional forms of fertilization. It should be used in addition to your normal fertilizing routine and won’t do any harm.

Other factors could be at play depending on what’s in your aquarium water to begin with, however, So be sure to test it on occasion to see what you’re putting on your garden.

But at the end of the day, it’s not something you are really going to need to worry about.


Owner of and also owner of actual Aquarium Gravel believe it or not! ;). Setting up beautiful aquarium sceneries and habitats since I was very young. Enjoy!