What Actual pH Should You Have In Your Aquarium?

All the time when I read about the water in a fish tank I see pH everywhere. Even when reading about fish you see pH mentioned within a range. That made me think:

What pH Should I Have In My Aquarium? You should have a pH between 5.5 and 8.5 depending on the type of fish you have. Most aquariums should be between 6.5 and 8.

It’s a quite big range that is acceptable, and it sure is a lot depending on what pH you should have in your fish tank. There is also a lot to consider when trying to manage the pH. All this is thoroughly gone through in this article.

What is pH?

pH is a scale that ranges from 0 to 14 that measures the sourness of the water, the acidity. It is very important for fish since they live in a certain acidity in their natural environment in the wild.

That is the pH level you should match in your aquarium.

What defines acidity is how concentrated hydrogen ions are in the water. The lower the scale (closer to 0) the more acidic the water is and the higher the more alkaline.

Neutral pH is when you get a measurement of 7 on the scale, it’s neither acidic nor alkaline.

Since the fish you put into your aquarium needs a certain pH, you should study what pH you need in your aquarium before buying fish. You should also check that the fish you are planning to put in needs the same pH.

Some fish want more acidic water (under 7) while others want more alkaline (above 7) and those are not good to put in the same aquarium.

Usually, you will find tropical fish that originates from South America and Asia likes more acidic water to neutral water. Fish originating from West Africa likes neutral water and fish originating from Central America and the deep lakes in East Africa likes alkaline water.

It’s not usually the wrong pH per se that kills the fish, it’s usually a quick change in the pH that will kill the fish. A change in pH could be because the aquarium is not taken care of properly.

You should always do regular water changes to keep the water quality top-notch!

But there is one factor you should take into consideration to prevent a sudden change in the pH, and that is the hardness of the water.

How hardness of the water is affecting the pH

The hardness of the water is affecting how easily the water can hold on to the hydrogen ions. When the ions are released the water gets lower pH and thus gets more acidic.

What water hardness is a measure of is how much minerals are dissolved into the water. It’s mainly calcium and magnesium salts that are the minerals dissolved. The more, the higher the hardness.

Most measure hardness in degrees of general hardness (dGH) which is fine but not very useful. A more useful measure is the carbonation hardness (KH) of the water.

That’s because in nature the rule is generally the higher the hardness, the higher the pH. In the tap water that we use to fill up the aquarium, the rule is in a lot of cases the opposite.

You should have control over the KH in your aquarium water. It’s easiest to have control when monitoring the hardness of the water with a KH test. You could have strips containing the measure, which works for most. But to get a more trustworthy result you need a testing liquid to measure the hardness.

On my recommended test page you will find what tests that I recommend measuring the KH with.

The KH should be above 3. Below 3 is considered soft water and pH will be more prone to vary and be a potential danger for your fish. If you have a KH value of 4 you will be fine with the pH balance but you should still monitor the KH because it could drop and be a problem for you in the future.

Change the KH

Generally, you want to raise the hardness and not lower it. There are some cases where you want to lower the hardness level of the water too, but those situations are not that common. They are when you have very hard water where you live.

To change the KH you could add calcium carbonate into the water. Another way to change the KH of water is to add seashells or coral into the filter.

It will release calcium into the water that gets dissolved (if the KH is low) and thus raises the amount of calcium into the water and that is giving a higher hardness level of the water.

To lower the hardness you could put peat into your aquarium making the water more acidic and therefore remove the minerals from the water, making it softer (lowering the KH).

How to measure pH in your aquarium

pH is easiest measured with testing strips. Although the strips are not completely reliable. A more reliable way to measure the pH is with specific measuring liquids. Those are generally more exact.

While using any water measuring you need to be sure the test is fresh. Take a look at the best-before-date of the package to be sure that it’s not too old. An opened package should be stored in a dark, cool place and be used up fairly quickly.

If the test is too old you run the risk of it showing the wrong result and that could give you a false feeling of safety. It could also make you think the parameters are wrong and doing measures that are not necessary or even kill the fish.

How often should you measure the pH you might think? Every week, while doing a water change or if you do any major change in the aquarium, like changing substrate or add new fish.

The more you test, the better since you will have control over the parameters and can detect any sudden changes and combat the changes quickly if necessary.

Now, you don’t have to measure every day, that quite excessive but once a week should be sufficient.

A problem is that it’s hard to detect if the pH is changing without testing the water. It is worth doing regular tests and you don’t have to do it with testing liquids. A testing strip is sufficient to see any sudden changes, even if it shows the wrong result of the actual pH. It’s the change that is important to monitor.

Testing the water is not a big cost of keeping fish as I have shown in this article where I count the cost of keeping fish. The cost of testing the water is about 6 cents per month counting during the lifetime of keeping an aquarium. That’s a low cost to be sure your fish are feeling well.

pH in your tap water

The tap water you fill your aquarium with has a certain pH. It’s not always the pH match with the one your fish need. The tap water is around the pH of 7, which is most likely okay for most fish.

The pH of the water varies a lot depending on where you live. At one place it could be 6.5 and at another place, it could be 8.3.

There are some easy ways to check the pH of the water.

  1. Check with your water supplier what the pH is
  2. Measure it yourself

To check the pH via the water supplier is probably the easiest way. Again, depending on where in the world you live it could be easier or harder to get this information. Most of the time you could go into their website and look at the water information.

Another way is to give them a call explaining that it’s important for you to have the information and they should be able to provide you with the information fairly quickly.

You could also measure the pH in your tap water yourself. You probably have testing strips at home or are about to go buy them, and you simply test the water as you do in the fish tank. But you do it with tap water instead.


As I was describing before, the hardness of the water is also connected with the pH. Basically how stable the pH is.

The carbonate hardness (KH) that we are interested in are often mentioned in soft, moderately, and hard water. The water supplier often mentions it in ppm or mg/l CACO3.

“How should I be helped with this!? I need it measured in KH!” you may ask. Well, it’s easy to convert. One KH is 17.848 ppm (or mg/l) CACO3.

If asking your water supplier what hardness you have on your water you will probably get the answer soft, moderate, or hard. The definition of these are:

  • Soft is 60-120 mg/l CACO3 = KH 3.4-6.7
  • Moderatly is 120-180 mg/l CACO3 = KH 6.7-1.0
  • Hard is more then 180 mg/l CACO3 = KH below 1.0

With our definition of one KH, it’s easy to convert. With soft water, you have a KH of 3.4-6.7. With moderately hard water the KH should be between 6.7 and 1.0. With hard water, you will get lower than 1.0.

But the easiest way of measure the KH, once again, is with the water test you use in your fish tank. Just measure the tap water and you will see exactly what is coming out of your tap.

Since the optimal KH for the pH to be stable is 4 or above you see that it’s most optimal to not have hard water. If you have hard water where you live you should try to change the water hardness by adding conditioner, making the water a little bit softer.

Some need a little more acidic water and other fish need a little more alkaline water. You might need to adjust the water pH or the KH, depending on the types of fish you have and the pH of your specific tap water.

How to lower the pH

The most common fish kept in an aquarium is tropical fish needing slightly acidic to neutral water. Most of the time you will be fine with the acidity of the tap water being neutral but in some circumstances, you need to lower the pH for your fish to be happy in the aquarium.

It’s always easier to alter the pH in soft water since the softer the water, the harder it has to keep the pH.

With soft water, you could add carbon dioxide (CO2) into the water to lower the pH. CO2 is the same thing as the bubbles in your soda and you might feel a little stinginess on your tongue when drinking something carbonated.

That stinginess is the acidity from the CO2 bubbling in your drink. Now, you should not pour any drink into your aquarium! There are solutions for adding CO2 into the water if you need a slightly more acidic environment for your fish.

With adding CO2 you need the right amount so the water is not getting too acidic for the fish. It could be hard to get the flow correct so perhaps an easier (but not as effective way) is to add peat moss into your filter.

Peat moss is a partially decayed plant that has decayed under lack of oxygen. It is acidic and will make the water more acidic when put into the filter.

Don’t go out and pick some peat moss from nature because that will color your water brown and make the aquarium very ugly to look at. You should go to a pet store and buy peat moss, be sure to pay a little bit more to get the good quality peat moss though.

A good quality peat moss will not color the water and it will be effective for a longer period of time. You see, low-quality peat moss has to change a lot more frequently than good quality peat moss.

You have to change the peat moss about every three months. With lower quality, it’s a lot more frequent. This is because the peat moss is decomposing and will lose its effect of lowering the pH that way.

If you have hard water or have to change it other ways than described above you could use chemicals to lower the pH.

If going with this route, be sure to use a chemical containing natural ingredients. As it always is, you should be careful to put something into the aquarium if it’s chemicals because it could change the balance in the aquarium quickly, killing everything inside.

When changing the pH, be sure to do it slowly. Sudden changes are killing fish so do it in steps and monitor the pH frequently when changing the pH, whether it’s chemically or with other, more natural methods. Once you found a balance in the pH that you are happy with, you should still monitor the pH to be sure the balance is maintained.

How to raise the pH

If you need to raise the pH in your aquarium because your inhabitants need more alkaline water, or if you have very acidic water in your tap water there are some solutions for you too.

One common way to raise the pH in the water is to add baking soda. Yes, you heard it right, it’s the same stuff you have when baking.

Generally, you could add 1 teaspoon per 5 gallons (20 liters) of water to raise the pH. Depending on how acidic and how much you have to raise the pH the amount could vary.

Another natural way to increase the pH is to increase the circulation of the water. That is making the CO2 release from the water. And as we know, CO2 is acidic (low pH) and with less CO2 dissolved into the water, the higher the pH.

This method is not recommended if you have fish that doesn’t want high water flow like labyrinth fish (bettas and gouramis for example) that want low water flow. Now, this kind of fish generally wants slightly acidic water so it’s quite uncommon to need to raise the pH if you have that kind of fish.

You could also raise the pH chemically. But as it is with lowering the pH, you have to monitor the pH carefully!

If changing it too quickly you will have problems. Also, you should never raise the pH without doing a water change!

That is because in water there are ammonia ions in the water. These are safe for the fish, but when raising the pH the ions will convert into ammonia which is very dangerous for the fish.

By doing a water change before raising the pH you remove the ions making it less likely to get an ammonia spike in the water.

It’s a bigger risk raising the pH than it is lowering it because of the risk of ammonia. So be careful and do a water change before. Also, do it slowly and you will be fine with raising the pH.

The sour fish

Most fish likes neutral to slightly acidic water. That’s because most of the fish we keep in aquariums are coming from waters around Asia and South America which are slightly acidic.

None of the fish need very acidic water. A pH from 5.5 to 7.0 is a common range among fish liking acidic waters. Neutral is good enough for most of these fish, but some don’t like neutral water and need it acidic, below 7.0

One common type of fish wanting a pH of 5.8-7.0 is rasboras.

Rasbora is a very popular fish coming in a lot of different colors and is easy to keep. The most common type of rasbora is probably harlequin rasbora.

They naturally live in Asia and don’t need a large aquarium.

Another fish that want slightly acidic water and are also coming from Asia is the gourami.

The gourami is also a very popular fish to keep. They need to breathe air and therefore they need to come to the surface to breathe. They live in very still water because of that.

As we know by now, the less movement in the water, the lower the pH (acidic). Gourami is coming in a lot of different sizes and colors. The pH the gourami need is 6.5 to 7.0.

A more uncommon fish is the hatchet fish.

The hatchet fish is needing a pH of 5.5-6.5. They have a big belly which makes them resemble a hatchet with their body shape.

The hatchet fish originate from South America and are prone to jump out of the water. Therefore they need to be kept in an aquarium with a sealed cover. They are carnivores and should be fed insects.

The alkaline fish

There are some common fishes amongst the ones liking living in alkaline waters too. Most common with these are the Guppy and Molly fish. These fish likes neutral water too so with keeping a pH of 7 you can keep these along with the fish liking more acidic water too.

There are also some fish not liking neutral water in this category too.

These fish are mostly coming from the deep water lakes in eastern Africa where are a lot of minerals dissolved into the water, making it more alkaline. Some fish in this category are also coming from northern South America or Central America where the water is more alkaline too.

Guppies like a pH between 7.0 and 8.5. They are probably the most common fish to be kept in an aquarium. Most people think of guppies when thinking of fish in an aquarium.

They come from Central America and northern South America.

They are beautiful to look at and comes in a lot of different colors and are easily available. They reproduce very easily. They are very easy to keep.

Another common type of fish that like a neutral to higher pH is the Molly fish.

The Molly fish is coming in a lot of different colors too. They need a pH of 7.0 to 8.5, just like guppies. Therefore they are commonly kept with each other. They are friendly to each other and the molly is easy to keep too.

They are also coming from the northern part of South America and Central America, just like guppies. In their natural environment, the two species are living together.

The molly will get a little larger than the guppy but still doesn’t need a large aquarium. An aquarium of 20 gallons should be sufficient for both molly and guppy.

African Rift Lake Cichlids are also a quite common type of fish, but with more experienced fish keepers. These really want alkaline water with a pH of 8.0 to 9.0. These are a group of cichlids living in the lakes in eastern Africa.

They are generally considered good-looking and are often very colorful. They are a little bit harder to keep due to their specific requirements of the aquarium.

They not only want high pH but they also (mostly) don’t want any plants. In their natural environment, they live in lakes which containing very few plants and mostly containing rocks.

If you keep these types of cichlids, it’s the only fish you could keep in that aquarium. They are not suitable in a community aquarium.


pH is very important for an aquarium. Well, it’s important for the inhabitants of the aquarium. When choosing what fish to get for your aquarium you should take a look at the pH requirements and see that they are the same between each type of fish.

Making a fish live in the wrong environment is animal cruelty and no one should make any animal suffer under any circumstances!

A tip to chose fish having the same requirement is to choose fish that live in the same part of the world naturally. The pH in the oceans varies depending on where in the world the ocean is. By choosing fish from the same area they will usually be compatible with each other.

Another important thing to have in mind is the hardness of the water. That is describing how easy it is to keep the pH stable. That is important because the fluctuation of pH is more damaging than the pH itself.

If you have a hardness level over KH 3 you should be fine by keeping the pH stable.

As with all water parameters, it’s important to keep monitor the pH to be aware if you get any spikes or drops in the pH. Especially if you doing any change in the water it’s important to keep track of every parameter, including the pH.

Related Questions

What happens if pH is too low in a fish tank? If the pH is too low the acidity of the water will corrode the skin of the fish making them die.

Will high pH kill fish? The high pH will make ammonia ions convert into ammonia which is very dangerous and will kill fish.


I have been a fish keeper a big part of my life. During the years I have gathered and searched a lot of information about fish keeping and here, on the website, is where I share that information to help you with your fish keeping. I want to provide the information that I didn't get when I had a question about my fish tank.

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