Hardness/GH

GH, or General Hardness, measures the concentration of calcium and magnesium ions in your aquarium water. These two minerals don’t affect pH directly, but they still play a big role in how your water behaves—and how your fish feel in it.

You’ve probably heard that hard water makes it tough to get soap to lather. While that doesn’t have anything to do with aquariums (please don’t put soap in your tank), it’s a familiar example of how mineral-heavy water behaves. In nature, GH often goes hand-in-hand with higher pH and KH, since mineral-rich environments tend to be alkaline. But in the home aquarium, GH can be altered independently, especially if you're using a water softener. These devices swap calcium and magnesium ions for sodium—technically softening your water, but not removing the dissolved solids entirely. And because GH is measured in parts per million (ppm), the swap doesn’t significantly impact salinity, which is measured in parts per thousand.

GH can be measured either in degrees of general hardness (°dGH) or ppm, with a rough conversion of 1 dGH ≈ 18 ppm. Most freshwater aquariums do well in the 4–8 dGH range, but depending on the species you keep, that number might need to be higher or lower.

Why GH Actually Matters

While calcium and magnesium don’t directly alter your pH, they’re essential for fish biology, invertebrate health, and plant growth. Many fish and shrimp, for instance, use calcium to regulate nerve and muscle function. In reef tanks, GH becomes even more important. Corals and other inverts rely on calcium and carbonate to build their skeletons, so maintaining a stable GH—alongside alkalinity and pH—is a key part of keeping a healthy reef.

Calcium and Magnesium: Getting the Balance Right

It’s not just about how much hardness is in your water—it’s also about the ratio. For freshwater tanks, especially when keeping sensitive species like Neocaridina shrimp, you’ll want a calcium-to-magnesium ratio somewhere between 2:1 and 5:1. A total GH between 4–8 dGH is usually a solid range.

Saltwater systems flip the script. In reef aquariums, you actually want more magnesium than calcium—something closer to a 1:3 or 1:4 ratio. Magnesium helps prevent calcium from precipitating out of solution too early, which is crucial for corals trying to form their structures.

Knowing what your water contains—and what your aquatic life needs—can make all the difference between just keeping fish and really thriving aquatic life.