A Fishy, Clever Way To Fertilize Plants


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For the last few years, I’ve been trying to find better and better ways of living in harmony with the environment.  Sure, I don’t have an electric car yet (I intend to get one though), but there are many other ways we can live healthily and try to work towards a balance.  One of those ways is to grow our own food. I really don’t have a lot of faith in corporations. This is especially true when it comes to supplying my family with healthy and safe food.  One of the best ways to make sure my family is getting a proper diet is to grow my own food. And finding a clever way to fertilize plants can not only help the plants, but it can also help your health.

Fish Tank Background

I like to keep fish.  I currently have 2 fish tanks in my house.  I’ve kept fish, reptiles, dogs, and cats for decades.  I even had my own pet store a few years back that I started from the ground up and then later sold.  A big part of that pet shop happening for me was the fact that I was breeding a large amount of fish.  In particular, African Cichlids were my specialty. And let me tell you, they eat a lot and thus need their water changed frequently.


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Keeping fish, if set up correctly, is both fun and relatively low-maintenance hobby.  I find sitting by one of my fish tanks, just watching the fish swim around is an immensely tranquil and meditative experience.  But you have to change some of the water every week or two, depending on how many fish, size of the tank and so forth. Now, I’ve made this maintenance ridiculously easy to repeat, making my life a lot easier.  Having the right tools helps too.

After you’ve cleaned your fish tank you will likely have a pail of brown water.  It smells and is generally unpleasant. Normally people would flush this brown water but there is much better use for it.  Fertilizing your plants, lawn or garden.

A plecostomus fish in the aquarium.
A plecostomus fish in the aquarium.

Fish Food 

Choosing fish food is almost as ridiculous as trying to find a healthy dog or cat food.  Regulations in most countries don’t exist when it comes to fish food safety. Many processed fish foods are stuffed full of what I’d consider some rather nasty chemicals.  So, when choosing fish, try to pick ones that you can feed as naturally as possible. If you go with prepared commercial fish food, make sure it’s clean and meant for feeding fish that will be used for consumption.  Not that I’m saying you’re going to eat your fish but keeping the whole system as organic as possible is the goal to provide the healthiest fertilizer for our plants.

Aquarium Chemicals

When considering that we are going to use the fish tank to feed our plants, let’s make sure that the food is clean.  This is especially important if you are intending to use the fish tank to fertilize plants that you are growing for food purposes.  I’ve kept fish for years with little to no need for chemicals. If you have a good ratio of fish to tank size and you are doing proper maintenance, then you shouldn’t have to use chemicals for your fish tank.  Keep it clean, keep it mean.


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Taking Out The Trash

In any of my fish tanks, I always like to ‘overdo’ the filtration.  I often run more than one filter, even two totally different kinds of filters for each tank.  One of the key things I do with my filters is utilizing activated carbon. Activated carbon acts like a sponge for many chemicals, drawing them out of the water.  It purifies the water by overpowering the attractive force on chemicals which makes them dissolve in water. The carbon exerts an even greater attractive force on molecules in water and thus ‘pulls them’ from the water.  This, in turn, purifies the water. Activated carbon is used for purifying drinking water for humans and has been a proven method of purification. I use it in my fish tanks to help keep the water clean and clear.

Pro Tip:  Best Aquarium Cleaning Kit Under $30 is a great little 3-in-1 kit including a small magnet scrubber, vacuum hose assembly, blade scrapers, and fishnet.  Okay, so the net is really not a cleaning tool, but this is a great package that you’

Fertilization Techniques

When it comes time to clean your tank, use your aquarium ‘vacuum’ and be thorough with cleaning the aquarium gravel.  Keep the brown water that will be in your pail after cleaning. The brown water is our clever way to fertilize plants. Here’s the thing though, you don’t want to use this brown water straight.  I recommend watering it down further as it is often quite concentrated. I use 50% tap water to 50% of the brown water from my fish tank. This ratio seems to work well with my gardens outside.

For indoor plants, I water down the brown water even further to 75% tap water and 25% brown water ratio.  I do this for two reasons. First, I don’t want to shock my plants. Indoor plants tend to be a bit more delicate than outdoor plants.  Also, I don’t want my house smelling like a dirty fish tank. So, I take the pail outside and pour some into another pail to water down.

Once I’ve fertilized all my plants, any left-over brown water is tossed onto the lawn.  The lawn has never looked greener or lusher and I don’t use any other kind of fertilizer on it.

When NOT To Use Fish Tank Water

Saltwater octopus from Metro Toronto Zoo, fall 2019. Picture by Jeremy Shantz

If all goes well, you’ll always be able to use your fish tank water as a fertilizer.  But, you won’t be able to at all if the tank is saltwater. Saltwater will kill many plants so never use it if you keep a saltwater tank.  That being said, saltwater aquariums are typically ridiculously expensive compared to freshwater and they also require a ton of extra knowledge and maintenance.  They are NOT for beginners and likely if you have a fish tank, you probably just have fresh water like most people who keep fish.

If you have a freshwater tank that you had to treat recently with chemicals for whatever reason, I’d hold off using the water to feed my plants.  This can be the case if you are treating a tank for ich or some other parasite that a new fish could have inadvertently introduced to your tank. I have not researched the possible effects of certain fish disease and parasite treatments on plants so I would simply say that I would give it a month after treatment and regular water changes before I proceed with using the water as fertilizer for your plants. 

Adventures In Aquaponics

Tilapia are a fish of choice for an aquaponics setup.
Tilapia – A clever way to fertilize plants.

Now here’s a concept I think is genius: Grow fish you can eat with plants you can also eat, using the fish to fertilize the plants.  Genius. What a concept. Tilapia is a common fish that is often used in aquaponics. And they are delicious to eat. They do well in warm water, 82-86 degrees Fahrenheit according to Worldwide Aquaculture dot com.  They are easy to keep and grow quickly, being ready to harvest and eat in as little as 6 months. You can keep them in pools at least 2 feet deep and their waste is an excellent fertilizer for many plants. And if you get the Tilapia to breed, you will only need to supplement the breeding group every couple of years with a small amount of fry to keep the genetics of the group in good order.

Although aquaponics is considerably more work to get set up, once running it’s like any other kind of gardening with a minor amount of extra maintenance for the fish. What a clever way to fertilize plants!


A decent article on aquaponics.

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