Next up in our beginner’s guide to water chemistry is KH.
With KH’s ability to directly impact the pH of your tank, you’ll want to watch it darn closely.
Today, I’m going to teach you everything you need to know about KH.
- What is aquarium KH?
- What is the difference between KH and GH?
- Why is KH important to your aquarium?
- What is the best KH level for your tank?
- Will increasing my KH also raise my pH?
- How do you test the KH of your aquarium?
- How do your increase KH?
- How do you decrease KH?
- Why should your KH levels NEVER reach zero?
What is aquarium KH?
Carbonate hardness is referred to as KH for short. It’s basically a measure of carbonates (CO3) and bicarbonates (HCO3) dissolved in your water.
Don’t worry! You don’t need to remember those words. You just need to know what KH is and why it’s important. And, that is actually simple…
KH is invisible. While it exists in your water, you won’t be able to know how much is there without a special test kit.
Think of KH as a protective barrier that surrounds your pH. As your aquarium creates acids, they eat away at KH instead of affecting your pH.
However, this barrier is not permanent and once gone, you risk your pH rapidly dropping.
The higher the KH of your aquarium, the more acid it can neutralize before the pH is affected.
You might also hear KH referred to as the alkalinity of the water.
- Alkaline: The opposite end of the pH scale to acidic (also referred to as basic).
- Alkalinity: The measure of acid neutralization (KH).
And just to confuse you more, you may also hear KH referred to as the following…
- Carbonate hardness
- Temporary hardness
- Total alkalinity
- Buffering capacity/buffer
- Acid-neutralizing capacity (ANC)
These can all be used interchangeably. So, if they ever come up in conversation, remember that they all refer to the same thing:
What is the difference between KH and GH?
Beginners often confuse carbonate hardness (KH) with general hardness (GH). While both have the word hardness in their name, they measure different parameters of your water.
Carbonate hardness (KH): The measure of carbonates and bicarbonates dissolved in water.
General hardness (GH): The measure of magnesium and calcium dissolved in water.
If you have ever heard someone say…
I have really hard water where I live.
They are talking about GH.
In nature, GH and KH go hand in hand. If a waterway has a high GH, it will also have a high KH.
But tap water is usually anything but natural…
It’s actually possible for your tap water to have a really high GH and a very low KH. So, just having a high GH does not automatically mean that you also have a high KH.
Why is KH important to your aquarium?
As I touched on earlier, KH prevents acids from causing your pH to swing.
Rapid changes in pH can shock and even kill your fish. So yeah, it’s something you want to avoid!
Now, here is what you might not be aware of:
Your aquarium constantly produces acids.
You know those beneficial bacteria that live in your filter, the ones you introduced when you cycled your tank? Well, they are to blame.
But here’s the thing…
Nitrite and nitrate are acidic!
And because they are constantly being produced, the pH of your aquarium will decrease over time.
If your KH is low or non-existent, then nothing can neutralize these acids.
This leads to an unsafe drop in pH that can make the water toxic for fish and plants.
KH is the invisible superhero in your tank that stops this from happening. As you see, it’s important to maintain some level of KH in your tank, no matter what species you keep.
In saltwater tanks, KH serves a second purpose. Corals use carbonates to build their exoskeletons. If you are creating a reef tank, you’ll need to watch that KH!
What is the best KH level for your tank?
The ideal KH levels entirely depend on what you stock in your tank.
Let’s look at the suggested ranges for different types of aquariums. While the measurements can be listed as part per million (PPM), I prefer to use degrees of carbonate hardness (dKH).1 dKH is about equivalent to 17.9 ppm
Please note, the following recommendations are rough guidelines only. Your specific fish, plants or invertebrates may require more precise KH levels outside these ranges.
|Tropical Fish Tank||4-8 dKH|
|Shrimp Tank||2-5 dKH|
|African Cichlid Tank||10-18 dKH|
|Planted Tank||3-8 dKH|
|Brackish Tank||10-18 dKH|
|Saltwater Tank||8-12 dKH|
|Reef Tank||8-12 dKH|
Will increasing my KH also raise my pH?
The two go hand in hand.
Generally, as KH rises so does pH. But don’t let this scare you away from maintaining a healthy KH.
Having a stable pH that is a little high is much better than a pH that swings up and down all the time.
How do you test the KH of your aquarium?
It’s important to know the KH level of your water.
Unfortunately, because KH is invisible, you’ll need a special aquarium test kit to do it…
Don’t worry! It’s affordable and easy-to-use. Best of all, a single KH test kit can last for hundreds of tests – just follow the instructions.
While there areaquarium test strips available that can also test for KH, I don’t recommend them. Based on personal experience, test strips are much less accurate than liquid test kits.
You may be wondering…
How often should you test the KH of your aquarium water?
Well, it all depends on the results of your test kit.
For freshwater tanks
- 4 dKH or lower: Check your KH weekly.
- 5 dKH or higher: Check your KH monthly.
You should test not only your tank but also your tap water. This will give you a greater understanding of how to go about adjusting your KH if you need to.
For saltwater tanks
I recommend testing your KH weekly. You should also check the KH of your salt mix, just to make sure everything is as it should be.
How do you increase KH?
Tested your aquarium water and want to raise your KH?
Let’s take a closer look at how you can do exactly that.
1. Water changes
Many water supplies across America have a KH high enough that performing a water change will replenish the KH levels in your freshwater tank.
Test your tap water and see if it is over 4 dKH. If so, a weekly 25% water change will replace the depleted KH.
Take this time to also maintain your tank. I highly recommend buying a good gravel cleaner and using it to suck all the gunk out of your substrate. Doing so will help prevent nitrates from building up and decreasing your KH.
As for saltwater tanks, a good salt mix should contain all the essential ingredients needed to restore the KH in your tank.
2. Alkalinity buffers
Many aquarium brands manufacture their own line of alkaline buffer products. Depending on the brand, they may rely on baking soda, soda ash or phosphate to increase KH.
So, why wouldn’t you use each of these individual products instead?
Well, the manufacturers of alkalinity buffers have spent a great deal of time ensuring consistency. These ingredients are mixed with other elements to ensure that you achieve your expected KH.
For beginners, I personally recommend using an alkalinity buffer over other methods in this list because it takes the guesswork out of adjusting your KH.
Best of all, there is an alkalinity buffer designed to match exactly what you stock in your aquarium…
Freshwater Tank Alkalinity Buffer
Marine Tank Alkalinity Buffer
Reef Tank Alkalinity Buffer
There are even alkalinity buffers designed for specific species of fish, including…
There really is an alkalinity buffer for everyone!
3. Crushed coral
Yep, this is exactly what it sounds like. Crushed coral comes from dead coral reefs. Because it is high in calcium carbonate, crushed coral can help boost your tank’s KH.
Best of all, you don’t need to add it to your tank constantly. You just let it sit there and do its thing.
How does it work?
Remember those acids I mentioned earlier in the guide?
Well, these acids react with the crushed coral, causing it to release calcium and carbonate into your water, raising both the KH and GH.
How much crushed coral should you use?
It doesn’t really matter. You see, adding more crushed coral will just cause it to increase the KH and GH faster.
However, given enough time, a small or large amount of crushed coral will eventually raise the KH and GH to the same point. Your water will equalize.
This is because as your pH rises, it becomes less acidic, and the coral won’t release as much calcium and carbonate.
I personally recommend adding crushed coral to your filter. If you add crushed coral to your gravel, it can compact over time, trapping fish waste, uneaten food and other gunk. If you do add it to your gravel, only use a small amount rather than laying down a thick layer. 
Aragonite is the crystal form of calcium carbonate (CaCO3). Just like with crushed coral, acids in the water cause aragonite to release calcium and carbonate.
However, unlike crushed coral, aragonite is made up of tiny sand-like grains. Because of this, it is often used as a substrate sand in both freshwater and saltwater aquariums.
Aragonite can greatly increase pH, GH and KH over time. So, it’s best suited to hard-water-loving fish like African cichlids.
I don’t recommend aragonite for beginners. If you decide it’s messing up your water parameters, you’ll have to remove the entire substrate – a frustrating job once your tank is set up.
5. Dolomite rock
Dolomite rock, sometimes called dolostone, is a calcareous rock made up mostly of calcium, magnesium and carbonate CaMg(CO3)2.
As you might expect, it will release all three of these elements into your aquarium, raising both the KH and GH of your tank.
Dolomite will release less of these ingredients at a higher pH. Because of this, dolomite is more commonly used in freshwater aquariums. The higher pH of saltwater tanks reduces the dolomite’s effectiveness.
Dolomite is available in a wide variety of colors, making it suitable to be used as a substrate.
6. Soda ash
Soda ash is sodium carbonate (Na2CO3). Because it can greatly increase pH, it is mostly used in saltwater tanks.
If you want to add soda ash to your tank, it’s best to add small daily doses rather than a larger weekly dose. This will give you a better opportunity to react to how it is affecting your pH.
How much soda ash should you use? Bulk Reef Supply has a great calculator that takes the guesswork out of dosing.
How do you decrease KH?
As I mentioned earlier, KH helps prevent your pH from dropping.
But what if the pH of your tank or tap water is too high for your fish or planted tank?
Well, to lower your pH, you first have to lower your KH.
And if your water has a naturally high KH, then you have to lower that before you have any success at reducing the pH.
While raising your KH is a simple task, lowering it is more difficult. A balance needs to be struck between KH and pH. Otherwise, you’ll experience pH swings that can kill your fish.
Decreasing KH is really only done in freshwater aquariums. Let’s take a closer look at the different ways to reduce the KH levels of your tank…
1. Acid buffers
To put it simply, acid buffers convert KH to carbon dioxide (CO2). The result is a reduced KH and a lower pH.
My number one tip when using an acid buffer:
Acid buffers are primarily used in planted tanks where the plants remove the extra carbon dioxide from the water.
However, if you don’t have plants, overdosing can result in both excess CO2 and a plummeting pH.
And, the end result is a bunch of dead fish.
So again, follow the instructions and take your time. You can always add more later.
2. Distilled water
Distilled water is water that has undergone a special process to make it pure…
The water is heated until it turns to steam, it is then passed through a cooler and collected in a separate container. All the impurities are left behind. The result is nearly pure water – no KH.
You can find it on the shelf of your local grocery store in gallon jugs.
Now, you want to mix this water with your tap water as you still want some KH and GH.
But by mixing it with say 50% tap water, it will adjust the KH accordingly. Use your aquarium test kit to determine the correct ratio.
The downside is the cost. Even though distilled water is fairly cheap per gallon, the costs can soon add up. Weekly water changes on a large tank can require a surprising amount of water.
For this reason, distilled water is best used for smaller tanks.
If you have a larger-sized aquarium, then my next solution is more appropriate.
3. RO/DI water
Make your own pure water with one of the following units:
- Reverse Osmosis (RO) System
- Deionization (DI) Filter
Both of these devices can be used to create “pure water” with no KH.
While the set-up costs may be pricey, it will save you a considerable amount of money in the long term, especially if you buy bottles of distilled water each week.
Just like distilled water, you want to mix RO/DI water with your tap water to ensure there is at least some KH and GH. The RO/DI water will reduce your KH proportionately to how much you mix in.
If you have a saltwater tank, it’s basically expected that you will have one of these units nearby.
4. Indian almond leaf
Indian almond leaf is a favorite for lowering the KH and pH of freshwater tanks. It’s particularly popular in betta tanks.
As the Indian almond leaves break down in your tank, they release tannic acid (tannins). These tannins eat away at the KH.
Indian almond leaves may also have medicinal properties, naturally protecting betta from skin issues and helping wounds heal.
For more information, check out FishLab’s detailed Indian almond leaf guide.
The downside to Indian almond leaves is that they gently lower the KH and pH of your aquarium water. If your water has a significantly high KH, you might not notice much of a difference.
5. Peat moss
Peat is dried and chopped peat moss. Put it in a mesh bag and place it inside your filter.
Like Indian almond leaves, peat leaches tannic acid (tannins) into the water to reduce the KH and pH a bit. Again, it’s best used where only a small reduction in KH is needed.
If you want to use peat to lower your KH, buy an aquarium-safe variety. Many types of peat are sold for use in gardening and are mixed with chemicals to cut down on mold, which could kill your fish.
Why should your KH levels NEVER reach zero?
For freshwater aquariums, you might have come across advice to lower your KH to zero.
A KH of zero is particularly dangerous because it can lead to unsafe drops in pH that makes the tank toxic to both fish and plants.
Ideally, you want to maintain at least some level of KH in your tank, no matter what type of species you keep.
I most commonly come across this advice for fish like discus or certain types of shrimp. And yes, it’s true there are plenty of fish that live in a natural environment where the water has a KH very close to zero.
But here’s the thing. Those waterways have a LOT of other variables that help keep the pH level relatively stable.
No matter how hard you try, you cannot consistently replicate that environment in something as small as your aquarium.
Unless you are an expert fish keeper, I highly recommend that your aquarium has some degree of KH.
Fish need a stable pH for long and healthy lives. KH helps you achieve just that.
For beginners, KH is one of the most overlooked and least understood elements of aquarium chemistry. And, I can’t help but feel that API not including a KH test in their master test kit is part of the reason.
But never fear because now you know everything there is to know about it.
As you see, KH plays a vital role in keeping your water parameters stable.
Without it, your pH would bounce all over the place, stressing your fish and causing all kinds of problems.
And if you want to adjust your pH, then you need to change your KH first.
Do you test your tank’s KH levels? Let me know in the comments below!