What General Hardness Is and Why It Matters

General hardness (GH) measures the amount of dissolved minerals—mainly calcium and magnesium—in your water. Fish and plants need these minerals to build bones, scales, and shells. If your tap water is too soft, your fish cannot absorb what they need, and plants struggle to grow. Raising GH is straightforward: you add minerals back into the water through products or water changes.

Different fish have different needs. Discus and tetras prefer soft water (GH below 4). Cichlids, goldfish, and livebearers need harder water (GH 8 to 15 or higher). Before you adjust anything, test your current GH and know what your fish actually require. Many aquarists raise GH only when their fish show signs of stress or their plants begin to fail.

Key Takeaways

  • Test your water's current GH using a liquid test kit or digital meter before making any changes.
  • The fastest way to raise GH is adding a commercial hardness buffer product like Equilibrium or Flourish GH directly to the tank.
  • Partial water changes with mineral-rich water (from a different source or treated tap water) will gradually raise GH over time.
  • Adding crushed coral, limestone, or aragonite to your filter or substrate raises GH slowly but continuously as water passes through.
  • Different fish species need different hardness levels, so match your adjustment to what your fish actually require.

Testing Your Current General Hardness

You cannot raise GH effectively without knowing where you are starting. Buy a liquid GH test kit—brands like API Master Test Kit or Salifert include GH testing. Follow the kit instructions exactly: fill the test tube to the marked line, add the reagent drops one at a time, and count the color changes. Each color change usually equals 1 degree of GH (dGH). Write down your result.

If you prefer not to test at home, many aquarium stores will test a water sample for free or a small fee. Some will also test your tap water straight from the tap, which tells you whether your source water is naturally soft or hard. This matters because if your tap water is already at the hardness your fish need, you may not need to adjust anything.

Using Hardness Buffer Products

Commercial hardness buffers are the fastest and most predictable way to raise GH. Equilibrium (made by Thrive) and Flourish GH (made by Seachem) are the most common. Both dissolve when ready and raise GH without changing pH. Follow the dosing instructions on the bottle—they usually tell you how many teaspoons to add per gallon to raise GH by a certain amount.

Dose the buffer into the tank during a water change or directly into the water column. Wait 24 hours, then test again to see if you have reached your target. If you overshoot, a partial water change with untreated tap water will lower it. These products are inexpensive and give you precise control, making them the best choice if you need to raise GH quickly or if your tap water is very soft.

Raising GH Through Water Changes

If you have access to water that is naturally harder than your tank water, you can use it for partial water changes. Some aquarists collect rainwater (which is soft) or use distilled water (which is very soft) and mix it with tap water to fine-tune hardness. Others use water from a different tap in their home if it comes from a different source or treatment system.

This method is slower than adding a buffer but works well if you are making regular water changes anyway. Change 25 to 30 percent of your tank water once a week with the harder water. Test every two weeks to track the change. Over a month or two, your GH will gradually rise to match the water you are using for changes. This approach is free if you already have access to harder water, but it requires patience.

Adding Mineral-Rich Substrate or Filter Media

Crushed coral, limestone chips, and aragonite sand dissolve slowly in acidic or neutral water, releasing calcium and magnesium. Place these materials in a mesh bag inside your filter or in a separate container with water flowing through it. As water passes over the material, it picks up minerals and raises GH continuously.

This method is passive and inexpensive—a bag of crushed coral costs a few dollars. The downside is that it works slowly and unevenly. Some parts of the tank may have higher GH than others, and you cannot control the rate precisely. It works best in established tanks where you are not in a hurry and want a gentle, ongoing increase. If your substrate is already sand or gravel, you can add a small amount of crushed coral to the filter instead.

Avoiding Common Mistakes When Raising GH

The most common mistake is raising GH too fast. Fish need time to adjust to changes in water chemistry. Raise GH gradually over a week or two rather than all at once. If you add too much buffer in one dose, do a partial water change when ready to bring it back down.

Another mistake is confusing GH with KH (carbonate hardness). KH is different—it measures buffering capacity and affects pH stability. Some products raise both; others raise only GH. Read the label carefully. If you are trying to raise GH without changing pH, use a product that raises GH only, like Equilibrium or Flourish GH.

Do not assume your fish need harder water just because they are struggling. Low GH is only one possible cause of stress. Test ammonia, nitrite, nitrate, and pH first. If those are fine and your fish are still sick, then test GH and consider raising it.

Knowing When to Raise GH

Raise GH only if your fish actually need it. If you keep soft-water fish like tetras or discus, and your tap water is naturally soft, leave it alone. Soft-water fish can become stressed if you raise GH too high. If you keep hard-water fish like goldfish or African cichlids, and your tap water is soft, then raising GH is necessary.

Signs that GH may be too low include stunted growth in plants, fish that refuse to eat, or fish with torn fins or pale coloring. These signs can also mean other things—poor diet, high ammonia, or disease—so test your water chemistry completely before you assume GH is the problem. Once you have raised GH to the right level for your fish, test it once a month to make sure it stays stable.

Frequently Asked Questions

How much will raising GH affect my pH?

GH itself does not directly change pH, but the minerals in GH (calcium and magnesium) can buffer water and make pH more stable. If you use a product that raises GH only—like Equilibrium or Flourish GH—pH should stay the same. If you use crushed coral or limestone, pH may rise slightly because these materials are alkaline. Test pH after raising GH to be sure.

Can I use baking soda to raise general hardness?

Baking soda raises KH (carbonate hardness) and pH, but it does not raise GH effectively. If you want to raise GH, use a product designed for it, like Equilibrium or Flourish GH. If you accidentally raise KH instead, your pH may shift, which can stress your fish.

How often should I test general hardness?

Test GH when you first set up the tank, after you make any adjustments, and then once a month during routine maintenance. If you are using crushed coral in the filter, test every two weeks at first to see how fast it is raising GH. Once it stabilizes, monthly testing is enough.

What if my tap water is already too hard?

If your tap water is naturally hard but your fish need soft water, use distilled water or rainwater for water changes to dilute the hardness. You can also use a reverse osmosis (RO) filter to remove minerals from tap water before adding it to the tank. This is more expensive than raising GH but necessary if your source water is too hard.

Will raising GH help my plants grow better?

Yes, if your current GH is very low (below 2), raising it will help most aquatic plants grow. Plants need calcium and magnesium. However, if your GH is already above 4, adding more minerals may not help unless your plants are also lacking other nutrients like nitrogen or phosphorus. Test your water and observe your plants before assuming GH is the limiting factor.