Understanding Aquarium GH

Quick Answer

Aquarium GH, or general hardness, measures the amount of dissolved calcium and magnesium in the water. GH affects fish health, growth, breeding, plants, shrimp and snails. The best GH level depends on the species you keep.

At a Glance

What Is Aquarium GH?

GH stands for general hardness. It measures dissolved minerals in aquarium water, mainly calcium and magnesium.

Water with fewer dissolved minerals is called soft water. Water containing more calcium and magnesium is called hard water.

GH is different from pH and KH. Water can have a high pH but a relatively low GH, or a low pH with a moderate GH.

Why Does GH Matter?

Fish absorb minerals from the water around them. Suitable mineral levels help support normal biological functions.

GH can affect:

• Fish growth and development
• Muscle and nerve function
• Breeding and egg development
• Live plant growth
• Shrimp molting
• Snail shell strength
• Overall fish health

How Is GH Measured?

GH is commonly measured in:

• Degrees of general hardness, written as dGH
• Parts per million, written as ppm

One degree of GH is approximately equal to 17.9 ppm.

A liquid GH test kit is usually the easiest way to measure aquarium general hardness.

General GH Ranges

These are general descriptions. The correct GH depends on the fish, plants and other aquarium animals you keep.

Fish That Often Prefer Softer Water

Examples include:

• Discus
• German Blue Rams
• Cardinal Tetras
• Many South American dwarf cichlids
• Some wild-caught Amazon fish

Many commercially raised fish can adapt to a wider range than their wild relatives, provided the water remains stable.

Fish That Often Prefer Harder Water

Examples include:

• Guppies
• Mollies
• Platies
• Swordtails
• African Rift Lake cichlids
• Many rainbowfish

Shrimp and snails also need enough calcium and other minerals for healthy shells and molting.

Signs GH May Be Unsuitable

Possible problems include:

• Poor growth
• Breeding difficulties
• Weak snail shells
• Shrimp molting problems
• Lethargy or stress
• Fish failing to thrive

These symptoms can have many causes, so test the water instead of assuming GH is responsible.

How to Test Aquarium GH

Use a liquid GH test kit and follow the instructions carefully.

Test GH:

• When setting up a new aquarium
• Before choosing fish
• After changing water sources
• When using reverse-osmosis water
• When shrimp or snails have shell or molting problems
• When attempting to breed sensitive fish

How to Raise Aquarium GH

GH can be increased with:

• Commercial remineralizing products
• Calcium and magnesium mineral blends
• Crushed coral
• Aragonite
• Certain mineral-rich rocks
• Mixing harder tap water with softer water

Make changes gradually. Test the water after each adjustment.

How to Lower Aquarium GH

GH can be lowered by mixing your source water with:

• Reverse-osmosis water
• Distilled water
• Deionized water

Do not use pure reverse-osmosis or distilled water without adding appropriate minerals. Fish, plants, shrimp and beneficial bacteria still require minerals.

Rick’s Experience

During my years keeping and breeding tropical fish and owning the FinTastik Tropical Fish store, I found that stability was usually more important than trying to force every aquarium toward a textbook number.

Before changing GH, consider whether your fish are healthy and whether your local water is reasonably suitable for the species you keep.

Frequently Asked Questions

Is GH the same as pH?

No. GH measures calcium and magnesium, while pH measures how acidic or alkaline the water is.

Is GH the same as KH?

No. GH measures general mineral hardness. KH measures carbonate hardness and the water’s ability to resist pH changes.

What GH is best for freshwater fish?

There is no single ideal GH for every fish. Match the water to the species you keep.

Can high GH harm soft-water fish?

Very hard water may cause problems for sensitive soft-water species, especially wild-caught fish or fish being conditioned for breeding.

Can low GH harm livebearers?

Yes. Guppies, mollies, platies and swordtails generally do better with adequate calcium and magnesium.

Does boiling water lower GH?

No. Boiling is not a practical or reliable method for lowering aquarium GH.

Rick Recommends

Useful supplies for monitoring and adjusting GH include:

• GH and KH liquid test kit
• Freshwater master test kit
• Aquarium remineralizer
• Reverse-osmosis water when necessary
• Water conditioner

Related Articles

• Aquarium Water Quality Guide
• Understanding Aquarium pH
• Understanding Aquarium KH
• Understanding Aquarium Ammonia
• Understanding Aquarium Nitrite
• Understanding Aquarium Nitrate

Final Thoughts

Aquarium GH affects fish, plants, shrimp and snails in ways that are easy to overlook. Test your source water, choose livestock suited to those conditions and make any necessary changes gradually.