Blue gravel siphon and a white plastic bucket

Where Do You Put Dirty Fish Tank Water? Recycle with Care

Fish tanks, especially in their early days, generate plenty of wastewater. Regular water changes are par for the course. Wondering where to put the dirty fish tank water is a question that rises to the surface quickly. 

Is it safe to water plants with it?

Or is it better to toss it down the drain?

Aquarium water is safe to put in plants, particularly on decorative species. If the water column has received strong treatments, the dirty water is best kept away from edible plants, especially when they are growing fruit already.

Throwing dirty fish tank water down the drain has to be done carefully so that no plants (and livestock) are discarded accidentally. Sad as it may be, throwing a baby fish can be less harmful than sending a foreign plant into the sewage. The latter can cause a lot of damage to the local flora if it starts growing in the wild.

Plants like the water hyacinth are aggressive invaders, but that’s not the only one.

So, generally, dirty fish tank water can go into plants and it can go down the drain. But there are caveats.

Let’s break them down.

Can I Use Aquarium Water for My Plants?

Using dirty fish tank water to keep your house plants alive and thriving is the most common solution. Compared to flushing it down the toilet, it reduces waste considerably.

What’s more, fish poop is very good for plants. Just like manure and other animal fertilizers, fish biowaste is rich in nutrients that can help house and garden plants thrive.

What about aquarium fertilizers? Well, yes, of course, they also work on terrestrial plants. Aquatic and “dry” plants have their differences, but they have far more similarities. Fertilized fish tank water will benefit your house or garden plants.

But here come the caveats.

The water column is often treated with conditioners to lower the pH, remove ammonia, balance out nitrites, and so forth. Sometimes fish are medicated as well. On the whole, these substances won’t harm your flowers, but it would be better to not water your garden plants with them. That’s particularly true when the plants are flowering and are about to — or already are — bear fruit.

Truth be told, this is more of a precaution than anything else. Plants are sturdy, powerful chemical reactors that transform all kinds of things, light even, into leaves and fruits. I couldn’t find any study to confirm whether watering them with treated aquarium water will harm you, but better be safe than sorry.

I usually pour some of the dirty fish tank water on my compost pile, to keep it moist. However, I never do that if I had treated the fish with medicine as antibiotics can wreck the composting process.

So, it is safe to use aquarium water for houseplants. Water your garden plants too, but only when the water column hasn’t received many conditioners or medicaments or the plants are far from producing fruit.

Can I Throw the Dirty Aquarium Water in the Toilet?

Dirty fish tank water can be flushed down the toilet. Arguably, this is the easiest way to get rid of useless water during regular maintenance.

However, pouring aquarium water into the sewage system comes with one major risk: ecological catastrophe.

Dramatic much?

Not enough, if you ask me.

Aquarium plants bring many benefits to our enclosed ecosystems. They make them prettier to the eye and contribute massively to stable water conditions and fish health. But in the majority of cases, the pants we use in our fish tanks are imported, exotic species. Should the small branch you flushed down the drain survive, it could wreak havoc in your country.

And that’s no joke. I mentioned the water hyacinth earlier — a beautiful, flowering water plant that makes a lot of sense for aquariums, as it is a floater. I really like it and put it in my fish tanks when I spend time in South America. But it has a storied history of conquest and destruction. So much so that sites like this one dedicate it the special attention it deserves.

European frogbit is another pretty floater that can and has destroyed native habitats.

In conclusion, yes, you can throw your dirty fish tank water in the toilet, but make sure it is free of plants. Modern sewage systems are quite unforgiving to living aerobic matter, but plants can be ridiculously resilient.

Can I Put Old Tank Water in a New Tank?

Pouring water from an established tank into a new aquarium is a one-time option for putting the dirty water to good use. It helps with starting the nitrogen cycle. The old water should be full of invisible lifeforms that will kickstart things in the new setup.

All the same, don’t use only dirty water for the new aquarium. At least 50% of the water should be new. I’d go 70% or 80% even and let the new tank cycle for at least four weeks before adding fish to it. It is always a good idea to add plants before any livestock, as they help with water equilibrium big time.

Another thing that will speed up the process is filter media. It can be a sponge, biofilter, or a handful of substrate. All of them are ripe with beneficial bacteria that help establish good living conditions in new tanks. Also, substrate or filter media will help with cloudy water, which is practically a given in the first few days of the life of any aquarium.

Discard Your Dirty Aquarium Water with Care

Throwing dirty fish tank water shouldn’t be done thoughtlessly. Water your plants or toss it on the compost pile, as long as the water is free of conditioners and antibiotics.

Pour it into the toilet, but filter it enough to ensure it has no plant matter.

Use a bit of it for a new tank but provide enough fresh water to stimulate its own “seasoning”.

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