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Fish tanks need regular water changes. But what do you do with all those litres of used tank water? Well, you don’t have to just pour it down the drain because, interestingly, it has other uses.
Waste water from freshwater fish tanks is highly beneficial to many plants. A lot of commercial garden fertilizers contain nutrients that are commonly found in the waste produced in an aquarium. Using waste aquarium water in this way is not only beneficial to your plants, it’s also an environmentally friendly way to reuse the water you have already paid for.
So even if you wouldn’t describe yourself as a dedicated gardener you can still put your waste aquarium water to good use.
But what exactly is in the used fish tank water that’s good for a lot of plants? And how should you use it?
Let’s find out.
What Is In Used Fish Tank Water That’s Good For Plants?
As anyone with a planted fish tank will tell you, the nutrients in the tank water feed the plants. And it’s not just the nutrients in the water itself that help the plants to thrive. It’s also the fish poop in the substrate (eg gravel) too.
But what exactly is in the tank water that’s good for plants?
There are a lot of nutrients in used tank water, several of which are beneficial to healthy plant growth.
Here’s some of the main ones:
- Potassium
- Nitrogen
- Phosphorous
- Ammonia
If you’re a keen gardener you’re likely to recognise these nutrients as some of the ingredients that are common in shop-bought plant fertilizers. So you’re going to be familiar with their benefits.
And as an added bonus, used fish tank water also contains microorganisms. These tiny, almost microscopic, creatures have fertilization benefits for plants too.
When you water your plants with tank water, any microorganisms that are in the water will die. I know this sounds nasty but they will die anyway if you throw the dirty water away. When the microorganisms go into the soil with the other tank water nutrients they will decay and produce beneficial nutrients of their own. Which means that your plants will get the benefit.
Is Fish Tank Water A Good Fertilizer?
As you can see from the list above, used fish tank water is full of goodies as far as plants are concerned. So, unless you keep exotic plants that have specific nutrient requirements, used fish tank water is a good general plant fertilizer.
Tank water also provides the moisture that all living plants need to survive and grow.
Then there’s the fish poop and leftover food that ends up in the bucket when you gravel vac the substrate. It’s all good stuff when it comes to giving your plants the nutrients they need to grow and flourish.
I guess the simple answer to this question could just have been ‘Yes’. But I always think it’s best to have a good idea about why you’re doing a particular thing. And it’s interesting too.
Is Fish Tank Water Good For Houseplants?
Just like garden plants many ornamental indoor plants need feeding regularly, particularly during the growing season. The nutrients in used tank water can be just as good for indoor plants as it is for the ones in your garden. So if you have potted indoor plants you can water and feed them using your fish tank water.
Is Fish Tank Water Good For Growing Fruit And Vegetables?
Personally I’ve never used fish tank water on any fruit or vegetable plants. To be honest I don’t grow my own but I know that a lot of people do. When I looked into whether tank water was safe to use on edible plants I found that opinions differed.
Some people say ‘No’ because they believe there’s a risk of contaminating the plants if you use fresh animal waste.
Others say ‘Yes’ because animal waste, fresh or otherwise, gives the plant the nutrients it needs to grow a healthy crop. Fresh horse manure springs to mind here.
If you’re not sure whether to use your fish tank water on your edible plants do some research and weigh up the pros and cons.
I did come across lots of articles about Aquaponics though. Which basically means that you grow plants, including edible ones, on the top of your fish tank. It’s a special setup that keeps the top of the plants out of the water with their roots in the water so the plants are hydrated. And the roots also feed the plant by using the nutrients produced in the tank water.
I found this a fascinating idea but unfortunately I only got an overview. So I’m going to take a closer look at what’s involved for a future article.
Should Fish Tank Water Be Diluted Before Using It On Plants?
When you buy a packaged plant fertilizer you have to follow the dilution instructions because it comes in a concentrated form. Fish tank water on the other hand is already diluted for you. So as a general rule you won’t need to water down the nutrients in your tank water before you use it on your plants.
An exception to this would be if your fish tank water is really dirty. And I mean seriously dirty. If the tank hasn’t had a water change for a long time the nutrients in the water will be more concentrated. This is not only bad for your fish, it probably won’t do your plants any favours either.
So, if for some reason, your tank water is heavy with nutrients and debris it’s best to dilute it with some clean water before using it on your plants.
Which Tank Water Is Bad For Plants?
There’s always an exception to any rule. And this goes for using fish tank water on your plants too. Not all tank water is going to be good for your plants.
But what are the exceptions when it comes to watering your plants with fish tank water?
Here’s a few pointers about what fish tank water you shouldn’t use
Don’t use your tank water on plants if:
- You keep a salt water (marine) aquarium. The salt in the water is likely to harm your plants
- You’ve used medications because your fish got sick
- You’ve added chemicals to adjust the water pH or to combat algae etc
- The water is extremely dirty and smelly because of a lack of tank maintenance
- Your fish are showing signs of disease/illness and you’re unsure why
These points are worth bearing in mind if you want to keep your plants healthy.
Since keeping freshwater fish tanks myself I’ve always used the tank water to irrigate my garden plants during the drier months of the year. It started off as a way to reuse the water instead of wasting it. Like most people I’m on a budget. So reusing the water I’ve already paid for is a bonus.
And I know now that I’ve been doing my plants a favour too.