What GH Is and Why It Matters in Your Tank

GH stands for general hardness, which 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 GH is too low, some species struggle to reproduce, develop weak skeletons, or show stunted growth. Shrimp and snails are especially sensitive to low GH because they need calcium to build their shells.

Different fish have different needs. Tetras and discus prefer soft water with low GH. Cichlids, goldfish, and livebearers like guppies and mollies do better in harder water with higher GH. Before you raise GH, check what your fish actually need—raising it when your tank doesn't require it can stress the wrong species.

GH is measured in degrees (°dGH or °GH). Most community tanks run between 4 and 8 dGH. You measure it with a test kit you can buy at any aquarium store for $10 to $20. The kit uses a liquid reagent that changes color based on hardness. Test your water before you start, so you know how much you need to raise it.

Key Takeaways

  • GH measures dissolved minerals your fish and plants need, and different species require different levels—check what your fish actually need before raising it.
  • You can raise GH by adding crushed coral, limestone, or commercial hardness supplements directly to your tank or filter.
  • Crushed coral works slowly and steadily, while liquid supplements raise GH faster but need more frequent dosing.
  • Test your water with a GH test kit before and after changes so you know whether you have reached the right level.
  • If your tap water is already hard, you may not need to add anything—test it first to see what you are starting with.

Adding Crushed Coral or Limestone to Your Tank

Crushed coral and limestone are the slowest and most stable way to raise GH. Both work by slowly dissolving in the water and releasing calcium and magnesium. You can buy crushed coral at any aquarium store, usually in small bags for $5 to $15. Limestone chips work the same way and cost about the same.

Place the crushed coral in a mesh bag and drop it into your filter or tank. The water flowing over it dissolves it gradually. Start with a small amount—about a tablespoon per 10 gallons—and test your GH after one week. If it has not risen enough, add more. This method takes time but is hard to overshoot because the dissolution rate is steady and slow.

Crushed coral also buffers your pH, which means it helps keep pH stable. This is useful if your water tends to become acidic, but it can be a problem if you are trying to keep pH low for species that need soft, acidic water. Limestone does the same thing. If you need to raise GH without changing pH much, use a liquid supplement instead.

Using Liquid GH Boosters and Mineral Supplements

Liquid GH boosters work faster than crushed coral because they dissolve when ready. Brands like Flourish Equilibrium, Thrive, and Salty Shrimp GH+ are common choices. You dose them by the milliliter according to the bottle's instructions, usually once or twice a week. A bottle costs $10 to $25 and lasts several months depending on tank size and how much you need to raise GH.

Liquid supplements let you raise GH without raising pH as much as coral does. They also give you more control—you can raise GH by small amounts and test between doses. The downside is that you have to remember to dose regularly. If you miss doses, GH will drift back down. If you overdose, you can overshoot and have to do a water change to bring it back down.

Follow the bottle's dosing instructions exactly. Most are designed for planted tanks or shrimp tanks, so the dose per gallon varies. Measure carefully with a syringe or the dropper the bottle comes with. Test your GH three days after your first dose to see how much it has risen, then adjust future doses up or down.

Mixing in Tap Water That Is Already Hard

If your tap water has high GH already, you may not need to add anything. Test your tap water straight from the faucet with your GH test kit. Many areas have naturally hard water—calcium and magnesium are already in the supply. If your tap water reads 6 dGH or higher, you may only need to do regular water changes to maintain GH without supplements.

If your tap water is soft but you need to raise GH, you can blend it with water from a different source. Some aquarists collect rainwater or use distilled water to lower GH when needed, but the reverse—mixing in harder water—is less common because most tap water is already harder than needed. Ask your local water utility for a water report, which lists mineral content. They usually provide it free online or by phone.

Hard tap water can also mean high pH and high alkalinity, which may not suit your fish. Before you rely on tap water to raise GH, make sure the pH and alkalinity are also in the range your fish need. If tap water is too hard or too alkaline, use a liquid supplement instead so you can control GH independently.

Testing and Adjusting GH Over Time

Test your GH at least once a week when you are first raising it, so you can see how fast your chosen method is working. Write down the date and the reading each time. After two or three weeks, you will see a pattern—whether you are rising too fast, too slow, or just right. Then you can adjust your method or dose.

Once you reach your target GH, test every two weeks during regular maintenance. GH does not change as fast as pH, but it does drift over time as minerals are consumed by plants and removed in water changes. If you see it dropping, add more supplement or coral. If it is rising, you may be overdosing—cut back or skip a dose.

Keep a straightforward log: date, GH reading, what you added, and how much. After a month or two, you will know exactly how much supplement your tank needs per week to stay stable. This takes the guesswork out of maintenance and prevents big swings that stress fish.

Avoiding Common Mistakes When Raising GH

The biggest mistake is raising GH too fast. Fish and plants adapt to gradual changes but can be shocked by sudden jumps. If you add a large dose of liquid supplement all at once, GH can spike by 2 or 3 degrees in a day. Raise GH by no more than 1 degree per week. If you overshoot, do a 25 to 50 percent water change with softer water to bring it back down.

Another mistake is not testing your starting water. If you assume your tap water is soft when it is actually moderately hard, you may add supplements you do not need. Test first, then decide what to add. This saves money and prevents overshooting.

Do not use table salt or aquarium salt to raise GH. Salt raises salinity and sodium, not the calcium and magnesium that GH measures. It can harm freshwater fish and plants. Use only products labeled for GH or hardness, or use crushed coral.

Choosing Between Methods Based on Your Setup

If you have a planted tank or a shrimp tank, a liquid supplement is usually the best choice because you can raise GH without raising pH too much, and you have precise control. If you have a community tank with fish that tolerate pH swings, crushed coral in the filter is simpler because you set it and forget it.

If you do frequent large water changes—more than 50 percent per week—crushed coral may not keep up, and you will need a liquid supplement to maintain GH between changes. If you do small water changes—25 percent or less—crushed coral usually works fine.

If your tap water is already moderately hard (4 to 6 dGH), you may only need to add a small amount of coral or skip supplements entirely and rely on water changes. If your tap water is very soft (below 2 dGH), you will need a regular supplement no matter which method you choose.

Frequently Asked Questions

Can I use baking soda to raise GH?

Baking soda raises alkalinity and pH, not GH. It adds sodium bicarbonate, not calcium and magnesium. Use crushed coral or a GH supplement instead. If you need to raise both GH and alkalinity, use coral. If you only need alkalinity, baking soda works.

How long does it take crushed coral to raise GH?

Crushed coral usually raises GH by 1 to 2 degrees over one to two weeks, depending on how much you add and how much water flow passes over it. Finer crushed coral dissolves faster than larger chunks. Test weekly to track progress.

Will raising GH hurt my fish?

Raising GH slowly—by no more than 1 degree per week—does not hurt most fish. Fish that need soft water, like tetras and discus, may struggle if GH gets too high, so check your species' needs first. Raising GH too fast can shock any fish, so go slowly.

Do I need to raise GH if I have plants?

Plants use some calcium and magnesium, but most planted tanks do not need high GH. Plants get minerals from fertilizers and fish waste. If your GH is below 3 dGH and plants are growing slowly, raising it may help. Test and observe before adding anything.

What is the difference between GH and KH?

GH measures calcium and magnesium (general hardness). KH measures carbonates and bicarbonates (carbonate hardness or alkalinity). Both matter, but they are separate. You can have high GH and low KH, or vice versa. Test both to understand your water fully.