Is Fish Tank Water Good For Plants? Everything You NEED TO KNOW!

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If you’re a plant parent with a fish tank, you may have wondered before if you can water your plants with your old aquarium water. After all, reusing water is certainly better than wasting it.

Turns out, fish tank water actually can be quite good for plants! Many home aquarium hobbyists use their unclean fish tank water to give their plants the hydration and nutrients they need to thrive. 

However, not all aquarium water is good for plants. To learn about which types are best for plants and when you should water, read on. 

Why Is Fish Tank Water Good For Plants?

Though dirty water in your fish tank is bad for your fish, many plants actually love fish tank water! This is because your dirty fish tank water contains nutrients such as (*):

  • Beneficial bacteria
  • Phosphorus
  • Nitrogen
  • Potassium
  • Ammonia

High amounts of these compounds in your fish’s water hurts them as they will struggle to breathe through their gills in unclean water. They produce these toxins in their waste, so breathing them in can make them quite sick. 

Your plants are a different story, though. You may notice that some of the compounds on this list are actually present in fertilizer!

When you feed your plants with fertilizer from your local home improvement store, three of the main ingredients are:

  • Nitrogen
  • Phosphorus
  • Potassium

This means that reusing your fish tank water is like giving them a nice dose of fertilizer, which results in lush, healthy plants. 

Phosphorus is the most important of these. Without phosphorus, plants wouldn’t be able to photosynthesize! (*)

To take you back to science class, photosynthesis is when plants use energy from the sun to make glucose, aka food. Without sun (or being able to use the sun), plants would starve to death. 

This is why nutrients are so important to plants!

To keep all the living things in your home happy and healthy, you should always remove these compounds from your fish’s water (to keep them from getting sick) and, if you choose, give them to your plants (to help them grow!). 

This is one of the reasons plants in areas with lots of fish are so fruitful. There are more nutrients for them in water with higher quantities of fish, which makes it a plant’s dream!

What’s not to love?

What Types of Fish Tank Water Is Best For Plants?

It’s important to note that not all fish tank water is good for your plants. You want to keep the below in mind before pouring that dirty water on your favorite houseplant!

The only water from your tank you should be using to water your plants is:

  • Freshwater
  • Changed regularly
  • Without harsh chemicals

If these characteristics apply to your tank water, you can use it to water your plants. Hooray!

If your tank water doesn’t have all of these characteristics, though, you’ll want to avoid using the water on your plants. Though it might feel nice to recycle that water, you could end up harming your plants if you use the wrong water.

You don’t want to kill that nice snake plant in the kitchen, do you?

Read on to learn more about these water requirements and why they’re so important to keep your plant alive. 

1. Only Use Freshwater

Not all water is made the same. That’s why you need to only use water from freshwater tanks to water your plants!

If you have a saltwater tank, you won’t be able to use this dirty water to hydrate your plants. This is because the high salt content can actually dry out and kill your plant, which you certainly don’t want. 

Make sure your tank is freshwater only and isn’t high in any specific compounds such as salt to keep your plants hydrated instead of diedrated. 

2. Water From Regular Cleanings

If your fish tank water is extremely dirty before you change it, you may not be able to use that water for your plants. Believe it or not, too much of a good thing is certainly possible when it comes to plant life.

If there are too many “good” compounds in the tank water, it could actually serve to dry out the plant, killing it quickly. 

If you paid attention during high school biology, you know this phenomenon is called “osmosis.” This means all those nutrients suck the water out of the plant, similar to drying out meat by curing it in salt. Yikes.

Water that is changed frequently is best as the nutrients are still present in the water, but it’s diluted enough to be healthy for the plant. If you miss a few cleanings, be sure to dispose of the dirty water for a few cleanings before returning to use it as plant water. 

3. Water Free From Harsh Chemicals

If you put any harsh chemicals in your tank, you don’t want to use that water on your plants, especially if the plants are for food!

Harsh chemicals include anything that drastically changes the pH of the water or treats the fish medically. If you water plants with these, it could kill your plant from pH changes or result in plants that contain that medicine. 

Nobody wants fish-medicated tomatoes for dinner. 

Make sure your tank is free of harsh chemicals before using tank water on your plants, especially if you’re going to eat them. It’s probably best to put fish poop water on non-edible plants anyways, though. 

Going Forward

The next time you have to change your fish’s water, consider whether there are any plants at your house that might want a healthy drink. If you water your plants with this water regularly, you’ll find yourself with a thriving garden in no time. 

However, be sure to remember which types of fish tank water are actually good for your plants. Make sure you don’t give them any salt water or anything treated with harsh chemicals!

If you’ve been using fish tank water to water your plants, let us know how it’s going below. If you have more questions related to your home aquarium, comment below and we may pick your question next time!

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