What Is Water Hardness?

Water hardness refers to the amount of dissolved minerals in your water, mainly calcium and magnesium. Imagine it like this: Your water is carrying little particles that your tank inhabitants either love or hate, depending on what they are. The more minerals, the harder the water. Fewer minerals? Softer water.

That’s where General Hardness (GH) and Carbonate Hardness (KH) come in. These two players are doing different jobs in your aquarium, and knowing how they work will help you keep your water in check.

General Hardness (GH)

Think of GH as the nutritional backbone for your fish and plants. It measures the levels of calcium and magnesium in your tank. Your fish rely on these minerals to regulate their internal processes, so too much or too little can throw them out of whack.

I remember a time when I set up a tank for some tetras. I thought everything looked perfect—crystal clear water, thriving plants, fish swimming happily. Or so I thought. After a couple of weeks, my tetras were not as vibrant as they should have been, and my plants looked like they were ready to quit on me. Turns out, the water was too soft. Who knew that was even a thing? Well, now I do. And after adding a bit of calcium, everything perked right back up.

Carbonate Hardness (KH)

KH is your water’s pH stabilizer. It measures how many carbonates and bicarbonates are floating around, which help buffer your water against any wild swings in pH. If you’ve ever woken up to find your fish gasping at the surface because the pH took a nosedive overnight, you probably have a KH issue on your hands. Trust me, I learned this one the hard way when I lost a few shrimp after a pH crash. It was a real lesson in humility, I will tell you that much.

With a good KH level, your water stays steady, and your tank stays happy. It is like having a seatbelt on your aquarium chemistry—things stay a little more secure.

Why Does Water Hardness Matter?

So, why should you care about water hardness at all? Well, let me tell you, the health of everything in your tank depends on it. Every species has evolved to live in specific conditions, and messing with their ideal water hardness is like forcing someone to live on fast food when they need fresh veggies. It is not going to end well.

Fish and Water Hardness

When it comes to fish, different species need different levels of hardness to stay healthy. If you have fish like Discus or Tetras, they are going to thrive in softer water. But if you have African Cichlids or Livebearers, they will want harder water. Putting a fish in the wrong hardness level can stress them out big time.

I once introduced some guppies into a tank that was more suited for soft-water species. Not one of my brighter moments. They started acting sluggish, and I had no idea what was going on. After testing the water, I realized the GH was way too low. They were basically living in water that did not give them the minerals they needed. A slow adjustment fixed the problem, but it taught me a valuable lesson—always check your water hardness before adding new fish!

Shrimp and Water Hardness

If you are keeping shrimp, especially Caridina shrimp, this is where things get a little more sensitive. These guys are picky when it comes to water hardness. Too high, and you will see all kinds of molting issues. Shrimp need the right balance to molt properly, and bad molts can end badly—trust me, I have been there, and it is not a pleasant sight. For more detailed differences between Neocaridina and Caridina shrimp, check out this guide.

I remember being so excited when I got my first batch of Caridina shrimp. I was ready to become a shrimp-breeding pro, but I was clueless about how important water hardness was. The first molting disaster hit hard, and I ended up losing more shrimp than I would care to admit. Once I adjusted the GH, things smoothed out, and my shrimp were back to shedding their shells like pros.

Plants and Water Hardness

You might not think about plants caring much about water hardness, but they do. Some, like Amazon Swords and Java Fern, prefer softer water. Meanwhile, others like Anubias and Cryptocorynes are more tolerant of harder water. If your plants are looking sad, yellow, or stunted, there is a good chance they are not getting the minerals they need.

In one of my early planted tanks, I had a beautiful Amazon Sword that started showing yellow leaves, and I could not figure out why. Turns out, the water was too hard for it. After softening the water a bit, the plant bounced back, and I felt like a gardening wizard. Sometimes it really is the little adjustments that make all the difference.

How to Measure Water Hardness

Measuring water hardness is not some complicated process. In fact, it is one of the easiest things you can do to stay on top of your tank’s health. All you need is a simple test kit, and you will be good to go.

Steps for Testing Water Hardness

1. Grab a GH and KH Test Kit – You can find these easily online or at your local fish store. For example, you could use the API GH & KH Test Kit.
2. Collect a Water Sample – Take some water from your tank in a clean container.
3. Follow the Instructions – Add the test solution drop by drop until the color changes. Count how many drops it takes; each drop equals a degree of hardness.
4. Read the Results – Match your findings to the chart provided with the test kit. That will give you both your GH and KH levels.

Testing regularly keeps things running smoothly. Even if your tank looks fine, checking the water hardness every couple of weeks is a good habit to get into. It is one of those things that helps you stay ahead of potential problems.

How to Adjust Water Hardness

Okay, so you have measured your water hardness, and it is not where it needs to be. Now what? Adjusting hardness might sound tricky, but it is not that bad. You just need to go slow—your fish, shrimp, and plants like stability, so any changes should happen gradually.

Increasing Water Hardness

• Add Crushed Coral – This is an easy fix. Just toss it in your filter or substrate, and it will slowly release minerals into the water, raising both GH and KH. You can get crushed coral here.
• Use a Commercial Buffer – Something like Seachem Equilibrium can raise GH without messing with KH. This works great in planted tanks where you need to keep things balanced.

I have used crushed coral in many of my tanks over the years, especially for African Cichlids. It is a solid go-to solution, and you really cannot go wrong with it.

Lowering Water Hardness

• Dilute with RO (Reverse Osmosis) Water – RO water has little to no minerals, so mixing it with your tank water helps lower hardness levels. It takes some trial and error, but it works.
• Add Peat Moss – This releases tannins, which soften the water. Just be prepared for your water to take on a tea-like color. Some people love it, some do not. Personally, I kind of dig the natural look.

When I was trying to keep Caridina shrimp, I had to dilute my tap water with RO water to get the perfect GH. It is a bit more work, but the results speak for themselves. The shrimp were happy, and I avoided any more molting disasters.

Common Problems with Incorrect Water Hardness

Ignoring water hardness can cause all kinds of problems in your tank. Here are a few you might run into:

• Fish Acting Stressed – Rapid breathing, hiding, or strange swimming patterns could mean your GH or KH is off.
• Shrimp Molting Issues – If your shrimp are not molting properly, check your water hardness first.
• pH Swings – Low KH can cause your pH to crash, which is a nightmare for fish and shrimp.
• Stunted Plant Growth – Plants need the right minerals to grow. If your water is too soft or too hard, you will see yellowing leaves and slow growth.

Keeping your water hardness in the right range prevents these issues and keeps your tank running like a well-oiled machine.

Frequently Asked Questions

How often should I test water hardness?

It is a good idea to check GH and KH at least once a month. If you are keeping delicate species like Caridina shrimp or Discus, you may want to test more frequently.

Can I use tap water in my aquarium?

You can, but test it first. Tap water hardness can vary widely depending on where you live, and it may not be suitable for your tank without some adjustments.