How to Soften Aquarium Water (9 Safe Methods)

Wondering how to soften aquarium water? This guide is for you!

Fish need optimal water conditions to stay happy and healthy. One of the major problems any aquarist has to contend with is the issue of hard water.

What happens when you find yourself straddled with this problem in your fish aquarium? The best option is to soften the hard water.

You can use various safe methods to soften hard water and make it usable in your fish aquarium. For example, peat moss, aquatic plants, Reverse Osmosis, and water-softening pillows can be used. The critical thing to consider here is to use a safe and viable method.

Any seasoned fish keeper will tell you that fish cannot survive in hard aquarium water. Indeed, even humans have their own issues with hard water.

For example, hard water is blamed for shortened lifespans of durable household appliances. It is also responsible for a myriad of health issues, such as hair and skin problems.

Let’s have a look at this issue in detail. But, first things first: what is hard water?

soften water aquarium

Basics of Aquarium Water Hardness

Pure water is free of minerals and other impurities. It is composed of one hydrogen atom and two oxygen atoms per molecule.

You get pure water when impure water evaporates, leaving the heavier molecules behind.

Water that comes to the earth as rain is not considered pure, because it already has some other molecules dissolved.

The rainwater is absorbed into the earth’s aquifers. It accumulates even more molecules by passing through streams, ponds, and rivers.

River water is likely to contain minerals such as magnesium, calcium, and carbonate. It is unlikely that water within the same table area will have the same mineral or chemical composition.

This is akin to saying that no two places on earth have the same water composition.

Depending on the amount of minerals water carries, its hardness can be classified as soft water, moderate, or hard water.

Soft water has the most negligible mineral content, while hard water is the most saturated.

The type of water (based on hardness) you introduce into the fish aquarium determines the aquarium’s chemistry.

This, in turn, directly impacts the health and general wellbeing of your pet fish.

In discussing the water hardness of the aquarium, we talk about General Hardness (GH). Basically, this refers to the magnesium and calcium content in the water.

The GH of the aquarium water is constantly rising, and you often need to take steps to tame it.

The Carbonate Hardness (KH) refers to the carbonate and bicarbonate in the water. The KH and pH levels are closely related, as one often affects the other.

Since fish use their swim bladders to process these minerals, it’s vital that you closely monitor water purity in the aquarium.

Why is Hard Water Such a Big Problem?

Water hardness is a big problem almost everywhere. For example, homeowners are concerned about the corrosive impact of water hardness.

If you have ever tried washing clothes using hard water, you know how laborious this exercise is.

Also, health specialists talk about the health issues brought about by this problem. Aquarists know of the disastrous effects of having hard water in the aquarium.

Hard water has lots of dissolved minerals, which have a substantial negative impact on almost everything they touch.

Hard water in the aquarium affects the growth and development of the fish. This means two things. One, your fish will not grow as fast as they should.

Two, their brain and neurological functions are affected, meaning they’ll be unable to properly carry out most biological processes, such as reproduction.

Some species of fish are more affected by water hardness than others. While some species can handle hard water, others need soft, slightly acidic water.

Most livebearers, such as platies, guppies, mollies, and swordtails, can tolerate hard water quite well. Other fish that thrive in hard water include archers and paradise fish.

Most tropical fish, on the other hand, need soft freshwater in the tank. As a result, they are adversely affected in a hard water environment.

How Does Hard Water Affect the Aquarium Ecosystem?

Prolonged exposure to hard water makes some fish species stunted. Although they may not die immediately, they are gradually affected as the hard water eats into their internal organs.

Not all fish will die from being exposed to hard water. However, most will be affected in various ways, the most common being stunted growth and poor brain development.

You’ll only realize your fish are affected when they don’t achieve their optimal growth size. Or when they start behaving erratically due to neurological disorders.

Additionally, hard water hurts the other elements of aquarium life. For example, the aquatic plants in the tank may be affected.

The plants may have everything they need for optimal growth but don’t respond appropriately. This is because of the bulkiness of the minerals these plants are subjected to.

Water hardness also directly impacts the pH level in the aquarium. The water becomes either too acidic or alkaline.

This can be detrimental to the health of fish sensitive to pH changes.

How is Water Hardness Measured?

Water hardness is determined in two ways: the General Hardness (GH) and the Carbonate Hardness (KH).

GH detects the levels of magnesium and calcium ions in the water. On the other hand, KH measures the alkalinity of the water.

The amount of carbonate and bicarbonate ions detected determines the alkalinity level.

The Degree of Hardness (dH) refers to the hardness of your fish tank water. It can be expressed as GH or parts per million (ppm).

There are five categories of water hardness you can detect using these parameters:

  1. Very Soft Water – Measures 00 – 30 GH or 0 – 50 ppm.
  2. Soft Water – Measures 30 – 60 GH or 50 – 100 ppm.
  3. Moderately Hard Water – Measures 60 – 110 GH or 100 – 200 ppm.
  4. Hard Water – Measures 110 – 220 GH or 200 – 400 ppm.
  5. Very Hard Water – Measures upwards of 220 GH or 400+ ppm.

How Can I Tell My Aquarium Has Hard Water?

You can rely on some standard techniques to determine whether the water in your fish aquarium is hard or soft.

Talk to the Local Authority

If your water comes from a public system, the local authority might know its hardiness accurately.

Most city authorities employ competent personnel in such matters. They regularly test the water to check whether it’s fit for human consumption.

If you are to use this water to fill your fish tank, you’ll know whether you’re dealing with soft or hard water.

DIY Testing

You can test the water using soap or your preferred detergent to verify whether it is hard or soft. Hard water does not produce scum in soap, while soft water easily does.

If you don’t see bubbles when you rub the soap in water, you’re dealing with hard water.

Use a Water Chemistry Test Kit

Water chemistry tests do much more than just test the water hardness. They test the entire spectrum of water quality.

A full-fledged water quality test kit can give you instant results about the water chemistry.

Alternatively, you may opt for a test kit that accurately collects the data, which you can send to a lab for interpretation.

Check on the Impact of the Water on Appliances

Okay, this is not a viable method, really. It is not even scientific!

However, you may hear it mentioned elsewhere that you can test the water quality by looking at its effect on electric home appliances and metals.

This is not a viable option as it is unlikely to give accurate results. As it’s always said, better safe than sorry!

How to Soften Aquarium Water

There are many ways of softening water though not all will give you the desired results. This section looks at the most effective ways of softening aquarium water.

#1 – Use Peat Moss

This effectively softens water because it does not interfere with your fish’s daily routine. Peat moss should be put on top of the substrate at the bottom of the tank and left to go to work.

It can also be placed in the aquarium filters.

Don’t expect instant results with this method. Peat moss works by gradually reducing the number of minerals and chemicals in the water until the water is clear and clean.

The good thing about using peat moss is that it is easy to install and cost-effective. On the downside, however, the results may not be as accurate as you’d hope for.

It may also be ineffective if you’re dealing with super-sensitive fish.

This method works best if the GH and KH levels are not too high. For example, if you are looking to change the hardness level from moderate to soft, peat moss will do a good job.

Boil the peat moss before introducing it into the tank to kill germs. Again, it’s best to use high-quality, non-additive peat moss products available in the market.

#2 – Growing Aquarium Plants

To first-time keepers, keeping live plants seems strange and weird. Many prefer plastic plants because of their bright colors and interesting designs.

But, to the seasoned aquarists, live plants are far much better than plastic ones for two reasons. One, plastic plants cause injuries.

Two, live plants have multiple benefits that make your fish happy and healthy. By adding live plants to the aquarium, you can solve many problems at once.

For example, these plants absorb the minerals and chemicals that make the water hard and store them in their leaves.

This effectively removes these elements from the water, making it safe for your fish.

#3 – Use of Water Softening Pillows

This is one of the best ways of removing minerals and chemicals from aquarium water. You simply deep these pillows into the tank water and leave them there for some time.

The water-softening pillows act more or less like aquatic plants. They absorb all the unwanted minerals, leaving the water soft and clean.

The good thing about using this method is that it is effective, safe, and has zero side effects. In addition, they reduce the hardness typical in tap water and remove heavy metals.

The only downside is that water pillows can be pretty costly. You may have to use hundreds of them yearly if the water change frequency is high.

#4 – Use of Catappa Leaves

Also known as the Indian Almond Leaves, Catappa leaves have many applications. For example, they have chemicals that fight off fungi and bacteria.

When applied to the tank water, these leaves transfer these healing qualities to your fish. As a result, your pets are protected from parasitic infections.

Equally importantly, Catappa leaves leach tannin into the water, thus gradually bringing the pH down.

Catappa leaves also absorb and neutralize calcium and magnesium ions, making the water safe for your fish pet.

Avoid putting too much Catappa leaves at once, as it may drastically lower the pH level.

On the flip side, Catappa leaves discolor the tank water.

#5 – Use of Reverse Osmosis

Many seasoned aquarists recommend this as the most effective way of removing unwanted minerals from aquarium water.

One thing about this method is that it softens the water without messing up your fish’s lifestyle. Another thing is that it removes pollutants and impurities from the source water.

Various Reverse Osmosis techniques are in use, each designed for different tank sizes.

Reverse Osmosis is such a massive success that it is used by the leading industries worldwide to soften water and their firms.

You need to combine Reverse Osmosis with tap water to achieve the desired results.

You’d expect such a popular and massively successful method to cost an arm and a leg. But, surprisingly, this is one of the most affordable methods you can use in your fish tank.

#6 – Use of Driftwood

Although not many aquarists recommend or apply this method, it is a choice you can consider. Driftwood works more or less like Catappa leaves.

It gradually releases tannins, thus lowering the pH to desired levels. Although driftwood changes the tank water color to brown, it has no side effects and will not harm your fish.

This method is cheap and readily available – you can actually get driftwood from your local pet store.

#7 – Use of Distilled Water and Demineralized Water

Both distilled water and demineralized water are readily available from local stores.

The difference between the two is that while distilled water has 0% minerals, demineralized water may have traces of minerals.

You may want to consider both or either option. However, filling your tank with either distilled or demineralized water is not cheap, as you’ll need gallons upon gallons.

#8 – Rainwater Harvesting

Rainwater harvesting is a good option if you can’t get reliable, clean water from public or private sources.

Indisputably, rainwater is purer in form than the water we find in the reservoirs on earth. This is because rainwater has not interacted with the minerals that cause water hardness.

However, due to the effect of air pollution, rainwater is becoming less desirable for human consumption.

You need to treat or distill it before you can put it into your drinking glass.

But it is without a doubt that rainwater remains one of the best sources of soft water for your fish aquarium.

Be sure to use the proper harvesting method to not contaminate the water.

#9 – Make Partial Aquarium Water Changes

This ranks the lowest in our list because of its unpredictable outcome. Making partial water changes solves some tank problems, but doesn’t have much impact on water hardness.

You create chemical imbalances in the aquarium ecosystem every time you take off some water and top it with fresh water.

Here are some disruptions you can expect if you opt for this method:

  • Loss of KH leading to pH alterations

Plants play an essential role in balancing the pH level in the tank. Without adequate KH buffering, the pH levels can swing wildly, putting plants and animals at risk of death.

  • Loss of fish slime

Slime is created by the oils fish secrete as they swim around the aquarium. Slime is an important component of the aquarium ecosystem as it kills dangerous bacteria.

The presence of slime also gives fish a sense of ownership; they feel at home. Unfortunately, making partial changes to the aquarium water leads to slime loss.

  • Loss of beneficial bacteria

Beneficial bacteria break down waste matter and prevent the production of toxins and poisons, such as ammonia and nitrites.

Changing the water, even partially, leads to a substantial loss of good bacteria. This means the tank becomes susceptible to an influx of parasites and toxins.

  • Introduction of pollutants

The new water you add to the tank may carry pollutants, such as pesticides and heavy metals. Unfortunately, this means that instead of solving the initial problem, you’re making the situation worse for your fish.

Before adding it to the aquarium, this is your cue to test any new water. In particular, be wary of tap and rainwater.


There are more than 9 methods you can use to soften aquarium water. Unfortunately, not all are safe for your fish.

We have left them out because they interfere with your fish’s growth and progress.

Before you settle on the method you think suits you best, consider whether you have a real problem with hard water. Consider the kind of fish species in your tank.

This is because some fish species will easily survive hard water. But, sadly, a good number may not last a day in a hard water environment.

Frequently Asked Questions (FAQs)

How Can I Test the General Hardness of Water?

You need a general water hardness test kit to test the General Hardness (GH) of water. This kit will measure the levels of magnesium and calcium in the water.

If the test results show a reading of 0 – 8 dGH, your tank water is soft. However, if the reading goes to 9 and above, you’re dealing with hard water.

How Can I Test the Alkalinity of Aquarium Water?

The alkalinity test is known as the Carbonate Hardness (KH) test. It measures the levels of carbonates and bicarbonates in the tank water.

A Carbonate Hardness (KH) test kit is the best way to test alkalinity. The alkalinity levels in the water determine the pH levels.

If the test shows high KH readings, the water will not alter its pH easily. This is good, for it discourages instability in the water.

On the other hand, low KH readings indicate that the water can change its pH levels erratically.

Can Fish Species Adapt to Hard Water?

A good number of fish species are good at adapting to hard water. For example, livebearers and paradise fish do relatively well in hard water.

However, these species need to be introduced to hard water gradually. A sudden change in water hardness will likely shock them and compromise their health.

Most tropical fish find it difficult to live in hard water. As a result, they need a longer time to get used to hard water conditions.

Don’t put a new fish in the main tank immediately after you bring it home. Instead, quarantine it in soft water and gradually increase the hardness.

This enables the fish to gradually adapt. Without this approach, your fish may die from improper fish transfer.

Remember, quarantining new fish also helps prevent the spread of disease to the old fish.

Should I Reduce Water Hardness Rapidly?

It would be a serious mistake to change water rapidly in a tank with fish in it. However, if you’re cycling a brand-new tank, this would not be a problem.

The best way to go about changing the water hardness is to do it gradually. This gives the fish time to adjust as they accept the new parameters you’ve set for them.

Changing the water hardness rapidly is like doing a 100% water change. Both will have a catastrophic effect on the fish.

It would be best if you strived to match the water hardness to what your fish are used to. This guarantees their safety and health.

How Can I Tell My Fish is Affected by Water Hardness?

While some fish cannot last a day in a hard water environment, most are affected over time. Actually, you may not know that your pet fish is suffering until it is too late.

Fish affected by water hardness experience stunted growth. Also, their internal organs are affected, making them unable to carry out vital biological functions.

This is why it is crucial that you test your water regularly for GH and KH. Using the correct test kits to do this will alert you in good time when things are not right.

Similar Posts