How To Soften Aquarium Water (5 Easy Effective Methods)

Water hardness may or may not affect you, depending on where you live in the world. The level of hardness of your tap water can adversely affect your aquarium, so you may wish to explore and ways you can soften the aquarium water.

If you’re not sure what water hardness is, it’s simply the level of naturally occurring minerals that are dissolved in the water.

Water hardness will generally fall into one of two categories, Carbonate Hardness, and General Hardness. We’ll look into the differences between the two later on in this article.

So, how can we soften aquarium water? We have five commonly used methods to reduce aquarium water hardness, these are Reverse Osmosis, Rain Water, Peat, Driftwood, and Water Softening Pillows. Each of these methods has advantages and disadvantages, so may not be suitable for all use cases. We’ll take a closer look at each of the water softening techniques below.

How to Tell If Your Aquarium Water Is Hard or Soft

There are a number of ways to tell if you’re aquarium water is hard, one of the easiest is to look for scaling within kettles or for scum or rings left in bathtubs and sinks after use. If you see or feel any white deposits, then chances are your water is hard.

A more scientific approach is to use a testing strip. These can be bought for only a few dollars and will give you a good indication as to how hard your water is by matching the test strip color to a chart. You can get them here.

Alternatively, you can call your local water company, and they should be able to tell you whether your water supply is hard or soft. There are also maps published online that will give you an idea if the water in your area is hard or soft.

What is Hard Water

General Hardness (GH)

Water with a general hardness rating (GH) will have both calcium and magnesium dissolved in it. The amount of dissolved minerals will increase the GH rating. Many fish species, but especially tropical fish, will struggle to thrive in water with hardness rating over a certain amount. The exact amount will vary from species to species.

Carbonate Hardness (KH) & (PH)

Carbonate Hardness (KH) differs from GH as its a measurement of the carbonate and bicarbonate ions present in the water. This level of carbonates also affects the buffering capacity of water.

There is a direct correlation between carbonate hardness amount and how much the PH level in an aquarium can fluctuate. The higher the KH level, the less likely the PH level is going to fluctuate. Conversely, the lower the KH value, the higher the chance of seeing fluctuations in the aquarium water PH levels.

If you’re having issues with the PH level in your aquarium, then you need to take a look at the Carbonate Hardness (KH) levels. The two are very closely linked, and changes to KH will impact PH.

Hardwater Fish Species

There are certain species of tropical fish that will tolerate hard water, so you can choose these varieties if you’d prefer not put up with the hassle of softening your aquarium water. Most of the livebearers such as Guppies, Platies, Swordtails, and Mollies can live quite happily in hard water. So will Paradise Fish, Cichlids, Scats, Monos and Archers.

If you’re wondering what species are suitable for hard water aquariums or better suited to software, then you can check out our guide to fish species. As a responsible pet owner, it’s important that you give your fish a comfortable life as possible and do your research before stocking a tank. Not taking the proper precautions can lead to the death of several fish in a very short amount of time, which is cruel as well as expensive.

Our end goal is to create a healthy environment for our fish to live in. Most of the time, tap water is fine, as long as chlorine and chloramine have been removed. We just need to avoid placing fish in water which is too hard.

When you’re doing your research on tropical fish species, keep in mind one rule. If the fish has only lived in soft water, then if you live in a hard water area, you’re not going to be your tap water without first treating it.

Even if the species of fish you’ve chosen is suitable for hard aquarium water, you’ll want to look out for signs of stress. A healthy fish population will be energetic, display the correct colors, and may even breed. If you see these signs, chances are your aquarium water is fine.

How To Soften Aquarium Water

It’s easier to tackle fluctuations in water hardness rather than start from scratch, so it’s important to measure it often. Testing strips are inexpensive and work pretty well, there are also electronic testers available which cost a bit more.

The methods to make hard water soft are not difficult, and can really make a difference to the safety and comfort of your fish. Just bear in mind, changing the hardness level in an active aquarium should be a gradual process. Any big swings in hardness rating can cause serious problems for your fish, causing illness or worse.

Reverse Osmosis

Reverse Osmosis (RO) is one of the most efficient ways of reducing water hardness and works without the use of any chemicals or harmful products. In fact, Reverse Osmosis will remove between 95 and 99 % of chemicals, minerals, and other dissolved particles, leaving behind pure clean water.

RO works by forcing the hard water through a slightly permeable material at high pressure. Any contaminants are left behind by the filter only allowing the clean water to pass through.

As RO can be a slow process, it’s often a good idea to process batches at a time, storing excess in a large Rubbermaid until you can use it next. You may find you need to mix RO water with regular tap water in order to find a balance between hardness and softness. This can help with balancing PH levels in an aquarium. Alternatively, you can also add specialist salt mixtures to the RO water.

Pro Tip: Record all the steps, quantities and outcome of your process. This will make it much easier when it comes to repeating the whole process all over again and ensure a consistent and repeatable approach.

While doing this, it’s important to maintain consistency. So please make it a point to write it all down. This way you can use the same technique and quantity for future applications as well.

Water Softening Pillows

If you’re asking yourself, what is a water softening pillow, then we’re to help. Water Softener Pillows use a chemical filtration media, normally an ionized resin that will reduce the general hardness of your aquarium water. They reduce the calcium and magnesium levels but will increase the sodium levels of your aquarium water.

The medium can often be reused and recharged by immersing the media in a salt solution for a period of time.

Most water softening pillows can be added to your existing water filter and are fairly effective at reducing GH. They are best used for smaller aquariums under 25 gallons but can be used in larger units, they’ll just need to be recharged more often.

One of the downsides of a water softening pillow is that they reduce the GH levels but increase sodium levels, which can in some circumstances be worse for your fish. It’s a trade-off, substituting one contaminant for another.

An RO unit is still a better buy if you have space and money.

Rain Water

If you’re looking for a super inexpensive option, then consider rainwater as an option. You’ll want to make sure you live in an area with decent air quality to guarantee chemical free water. Your water capture system and storage containers should be clean and sterile, as well as being food grade in order to avoid chemicals leaching into the water.

As the rainwater will be very soft, you may need to mix it with some tap water or aquarium salts in order to increase the KH and GH levels to the desired level.


Peat moss can be used to soften your aquarium water. It works by binding calcium and magnesium ions, but it will also release gallic and tannic acid into the water. These acids will then attack and degrade and bicarbonates in the aquarium, further reducing the carbonate levels and the waters PH.

There are a few ways to go about using peat in your aquarium.

  1. Place the peat inside your aquarium filter. This will ensure that a constant stream of water passing over the peat and maximize the effectiveness.
  2. Another way is to add peat to a bucket of water for at a minimum of two weeks before you intend to use it. When you’re ready to use the treated water in your aquarium, simply strain the water through a fine sieve and you’re good to go.
  3. lastly, we can add peat to a pillowcase or something similar and submerge it directly into the aquarium. You’ll need to make sure the tank is well aerated to keep the oxygen levels from dropping.

Whatever method you decide to go with, make sure you sterilize the peat first by boiling it for a few minutes. Additionally, only buy untreated peat with no additives.

A drawback of using peat is that it can turn your water brown for a while, this is the tannins released by the peat and is completely harmless, but can look unattractive, it will disappear after a time.


Driftwood works in a similar way to peat and will help reduce GH levels and PH levels.

It’s probably better to buy driftwood from your local fish shop, or even online. This will help avoid parasites and invasive species from infesting your aquarium. You can still boil your bought driftwood to ensure a clean start and avoid any nasty surprises.

If you submerge the driftwood in your aquarium straight away, you’re going to see your water turn brown. Much like peat, this is tannins being released by the wood. It’s harmless to fish, but might be something you want to avoid. One method to do this is to soak the wood for a couple of weeks before you use it.

Once you’ve added the driftwood to your tank, make sure the water is well aerated and moving. Within the first couple of weeks, we recommend keeping an eye out for any growth on the wood such as slime algae, frequent water changes can go some way to preventing this.

Final Thoughts

You don’t have to have a chemistry degree to understand this article. Most aquarium owners need only to know the basics to have a successful aquarium.

We think there’s a couple of things worth noting:

  • It’s easier in the long run to stock fish that tolerate hard water rather than change the hardness ratings.
  • It’s easier to make water which is too soft harder rather than making hard water soft.
  • A Reverse Osmosis unit is the best option if you want a consistent approach with very few downsides.
  • It’s important to constantly monitor the hardness and PH levels of your aquarium.

If you’re already successfully tackling hard water in your aquarium, we’d love to hear about your success. Let us know in the comments below.

The better the information we all have, the healthier and happier are fish are going to be.


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