Our final chapter in our beginner’s guide to water chemistry covers GH.
GH is essentially the hardness of your water.
And today, I’m going to teach you everything about it.
- What is aquarium GH?
- What is the difference between GH and KH?
- Why is GH important to your aquarium?
- What is the best GH for your aquarium?
- How do you test the GH of your aquarium?
- What happens if your GH is too high?
- How to raise aquarium GH – increase hardness
- How to lower aquarium GH – decrease hardness
What is aquarium GH?
General hardness, also known as water hardness, is referred to as GH for short. It’s basically the measure of the many salts that are dissolved in your water. In particular, calcium and magnesium.
Water with a low GH is said to be soft, and water with a high GH is considered hard.
So, soft water has very little or no calcium and magnesium. Hard water has a lot.
While GH is an important part of water chemistry, most of you using tap water don’t need to worry about it.
You see, your GH entirely depends on your water source. Most tap water generally has sufficient GH so that you’ll never have problems with it.
There are generally two reasons you need to monitor the GH in your aquarium:
1. You have either very soft or very hard water
If your water source is very soft or very hard, keeping certain types of fish that prefer to live in the middle of this range can be difficult. You’ll need to keep an eye on your GH and raise or lower it accordingly.
2. You want to keep a specific type of fish
On the flipside, some fish thrive in hard water such as cichlids, while others thrive in soft water like discus. If you want to keep these fish, you need to monitor your GH.
It is also worth mentioning that since saltwater tanks use RO/DI water, they don’t have to worry about GH because the salt mix restores these minerals.
What is the difference between GH and KH?
Beginners often confuse general hardness (GH) with carbonate hardness (KH). While both have the word hardness in their name, they measure different parameters of your water.
General hardness (GH): The measure of magnesium and calcium dissolved in water.
Carbonate hardness (KH): The measure of carbonates and bicarbonates dissolved in water.
If you’ve ever heard someone say…
I have really hard water where I live.
They are referring to GH.
KH, on the other hand, keeps your pH levels stable.
In nature, GH and KH go hand in hand. If a waterway has a high GH, it also has a high KH.
But tap water is usually anything but natural…
It’s quite 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.
For more information on KH, check out FishLab’s detailed guide.
Why is aquarium GH important to your aquarium?
The number one reason why you should monitor the GH of your aquarium comes down to one word…Osmoregulation.
Osmoregulation is a natural process that your fish do automatically. It’s like you or me breathing, meaning it just happens.
To put it super simply, osmoregulation is the process of your fish balancing the salts and water inside their body with the salts and water outside their body.
If an imbalance occurs, it can stress your fish or worse. In severe cases, it can lead to the death of your fish.
This is why a freshwater fish won’t survive in salt water and vice versa.
There is another reason why GH is important…
You’ve probably seen this word in Gatorade advertisements.
Electrolytes is just a fancy term for minerals and salts that conduct electricity when dissolved in water – minerals like calcium and magnesium (GH!).
These electrolytes aid in muscle and bone growth, digestion, development of gills, improves resistance to diseases, and more…
Pretty darn important stuff, right?
Your fish need electrolytes to remain healthy, and the only place your fish can get them is by drawing them from the water.
But here’s the thing…
Water with no GH has no electrolytes for your fish to use!
This can lead to some serious health issues.
Attention planted tank owners: Even aquatic plants need some degree of electrolytes. For example, plants use calcium to grow.
If you have very soft water, then the GH might not be high enough to supply your plants and fish with the electrolytes they need to remain healthy!
As you see, it’s pretty important that your aquarium contains at least some level of GH.
You should also note that the GH of your aquarium can change over time. GH can become lower in your tank because fish and plants use up the trace minerals.
Or, it could be that GH is higher in your tank because calcium and magnesium are left behind when water evaporates from the tank.
What is the best GH for your aquarium?
The ideal GH 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 general hardness (dGH). One dGH is about equivalent to 17.9 PPM.
Please note: The following recommendations are rough guidelines only. Your specific fish, plants and invertebrates may require more precise GH levels outside these ranges.
|Tropical Fish Tank||4-12 dGH|
|Shrimp Tank||4-8 dGH|
|African Cichlid Tank||12-20 dGH|
|Discus Tank||3-8 dGH|
|Planted Tank||3-8 dGH|
|Brackish Tank||12-20 dGH|
|Axolotl Tank||7-14 dGH|
How do you test the GH of your aquarium?
The important thing to remember is that the GH of your aquarium is invisible. You won’t be able to tell the difference between soft water (low GH) and hard water (high GH) just by looking at it.
To determine the GH of your water, you have to test it. And to do that, you’ll need an aquarium test kit. This one, to be exact…
Don’t worry! It’s affordable and easy-to-use. But best of all, a single GH test kit can last for hundreds of tests. Just follow the step-by-step instructions for accurate results.
While there are aquarium test strips available that also test for GH, I don’t recommend them. Based on personal experience, test strips are much less accurate than liquid test kits.
Now, you may be wondering:
How often should I test the GH of my aquarium?
Unless you have very hard or very soft water or want to keep a sensitive species of fish, most of you won’t need to worry about regularly testing your tank for GH.
However, if you do, I recommend testing it once every time you perform aquarium maintenance, such as a water change.
What happens if your GH is too high?
As I covered earlier, all tanks need some degree of GH.
But what happens if the GH of yours is too high?
Let’s look closer at three negatives brought on by high GH.
1. New fish can die
Okay, so this point is the matter of some debate. After all, you don’t have to go far to find resources that state a high GH will not affect fish as long as they have acclimated to the hard water.
Let me repeat that last point:
…as long as they have acclimated to the hard water!
But what happens if you buy a new fish and your aquarium has a significantly higher GH than that of your local pet store?
Well, that’s when things can turn nasty.
Generally, when you buy a new fish for your tank, you bring it home, float the bag for about 20 minutes, and then add the fish to your tank, right?
You have not given your fish a chance to adjust.
Your fish will experience what is called osmotic shock. While it can take a few days, osmotic shock can kill your new fish.
Because osmotic shock isn’t an instant killer, beginners often blame the death on the fish being unhealthy rather than their hard water.
Fortunately, there is a simple way to get around this…
Drip acclimatization is exactly what it sounds like. You drip water from your current tank into a container with the water your fish came in.
By adding your harder (or softer) water drop by drop, your fish will find it much easier to adapt to the change in water hardness.
It’s a slow process, but it gives your fish the greatest chance of survival.
Want to try your hand at drip acclimatization? I highly recommend grabbing this tool…
It’s almost like a gravel vacuum, but instead of streaming a constant flow of water, it removes it drop by drop and really simplifies drip acclimatization.
The reason? In their natural habitat, these fish breed during the rainy season. As their homes flood with extra water, the GH drops.
The fish use this drop in GH (among other factors) to let them know that it’s time to start getting frisky.
So if your GH is too high, then you will have difficulty breeding these sensitive fish.
3. Live plants
Most plants can tolerate a moderately high GH.
However, there are sensitive softwater plants that can be almost impossible to grow in hard water. Chain Sword or Dwarf Sagittarian are two plants that thrive in soft water and struggle when the GH rises.
The solution is simple: Select plant species that will happily grow in the GH of your water!
How to raise aquarium GH – increase hardness
Tested your aquarium water and discovered that you need to raise your GH?
Let’s look at how you can do exactly that!
1. Water changes
Is your tap water moderate to very hard? For most of you, that will be all that is needed to increase the hardness of your aquarium water.
Simply keeping up with your water changes will be enough to replenish the GH that has been depleted by fish and plants in your tank.
But what if you have very soft water? Or maybe, you just want to increase the GH higher than what your tap water currently contains?
Well, that’s where the following come in…
2. Water remineralizer
RO/DI water is very soft water that has been stripped of all its mineral content. It contains zero GH and, on its own, is unsafe for fish and plants.
Remineralizer restores the lost minerals. But it’s not just great for RO/DI water, you can use it to raise the GH of soft well water or tap water as well!
My favorite remineralizer is… Seachem Replenish.
By following the included instructions, this remineralizer allows you to reach a specific GH with little effort.
Seachem Replenish contains sodium chloride, so it is not suited for planted tanks or salt-sensitive fish, such as certain breeds of catfish.
Fortunately, Seachem also makes a remineralizer that is safe for plants and salt-sensitive fish too…
Free from sodium chloride, it’s perfect for raising the GH in your planted tank.
Remineralizer is generally added each time you perform a water change.
3. Wonder shells
Wonder shells contain magnesium and calcium. When added to your aquarium, these shells slowly dissolve, releasing these elements into your aquarium and raising your GH.
Wonder shells also condition your water, removing chlorine and chloramine from tap water and can help reduce cloudiness as well. They are particularly popular in small beginner aquariums.
Just be mindful that it will take some trial and error before you know how many shells you need to reach a specific GH in your tank.
4. Crushed coral
Crushed coral is exactly what it sounds like. Coral that has been harvested from dead reefs.
Because coral is largely calcium carbonate, it slowly releases calcium and carbonate into your water when added to your tank.
The one thing to be mindful about crushed coral is that it will raise not only your GH but also your KH. The higher KH means that your pH will also be higher.
It’s these exact characteristics that make crushed coral a popular choice in cichlid tanks, which prefer hard water with a high pH.
Aragonite is available as rocks or sand. Like crushed coral, it is made up almost entirely of calcium carbonate. When added to your aquarium, it releases both calcium and carbonate into your water.
Aragonite is mostly used as a substrate for hardwater-loving fish like African cichlids. It’s more pleasing to the eye than crushed coral. You might also hear aragonite sand referred to as cichlid sand.
Just like crushed coral, adding aragonite to your tank will not only increase your GH but your KH and pH too.
Some brands mix both aragonite and crushed coral together in one product to make it more visually appealing.
How to lower aquarium GH – decrease hardness
Okay, so decreasing the GH is slightly more difficult than increasing it. If you have hard water, you may find it easier to keep hardwater fish than plants.
Let’s take a closer look at the different ways to lower the GH of your tank.
1. Reverse Osmosis/Deionized water (RO/DI)
If you need to lower your GH every time you perform a water change, an RO/DI system is your best bet.
To put it super simply, this device removes salts, minerals and other contaminants leaving you with 99% pure water.
Best of all, this water is soft. So soft, it has a GH of zero.
So with your RO/DI water on hand, you have two choices:
1. Remineralize – Start from scratch and raise the GH to your desired level using a remineralizer like Seachem Replenish.
2. Dilute – Use the RO/DI water to dilute your tap water, lowering the GH down to your preferred level.
While it may be expensive to set up initially, an RO/DI system saves you money in the long run.
The other option is to buy bottles of…
2. Distilled water
Distilled water is essentially water that has been turned into steam then collected. When water turns to steam, it leaves behind salts and minerals, resulting in pure water.
Like RO/DI water, distilled water is soft and has a GH of zero and cannot be used on its own. It must either be remineralized or mixed with your tap water to increase the GH.
3. Peat moss
Peat is dried and chopped up sphagnum moss and, when added to your tank, can be used to soften your water.
Simply add peat to a mesh bag and place it inside your filter. As the water passes through the peat, it traps the GH minerals like calcium and magnesium. By removing and replacing the peat, you remove these minerals and soften your water.
There are some downsides to using peat. Namely, it’s not a very precise method of lowering GH, and its impact can be virtually unnoticeable on hard water with a high GH.
Peat also releases tannins, which can lower your KH and pH as well. These tannins also dye your water a yellow color. Depending on what you stock, this may not be desired.
Some people use Indian almond leaves to similar effect.
For those with soft or hard water, understanding and adjusting GH is an important part of fishkeeping.
While many beginners get by without testing their GH at all, there is a good reason why you should – the health of your fish are at stake!
Besides, as you see from this article, testing, raising and lowering the GH of your tank isn’t that difficult anyway.
Do you test your tank’s GH levels? Let me know in the comments below!