General Hardness

While most drinkable water is soft enough to support fishkeeping, water hardness can vary significantly depending on your local source—and that can make a big difference for your tank inhabitants. GH, or general hardness, refers to the concentration of calcium and magnesium ions in the water. These minerals may not affect pH directly, but they play a vital role in fish health, osmoregulation, and even the health of plants and invertebrates.

Different species of fish have different mineral requirements. For example, softwater fish like discus and cardinal tetras thrive in water with minimal dissolved minerals—typically around 0–3°GH. On the opposite end, African cichlids are native to highly mineralized waters and do best in a GH range of 8–18. Most common freshwater aquarium fish, however, do well with a GH of 4–8, which ensures there’s enough calcium and magnesium for general biological processes.

In saltwater aquariums, GH isn't typically measured as such—but it’s still critically important. Reef tanks in particular require elevated levels of calcium (around 400 ppm) and magnesium (around 1300 ppm) to support coral growth and maintain system stability. That equates to about 94°GH, which is one reason marine aquarists tend to measure calcium and magnesium separately instead of relying on general hardness.

The Importance of Calcium and Magnesium Balance

Getting the right levels of calcium and magnesium is important—but so is getting the right ratio. In freshwater systems, a calcium:magnesium ratio of around 2–5:1 is ideal. Shrimp, like Neocaridina, are especially sensitive to mineral imbalances. Even if the tank is clean, stable, and otherwise healthy, improper GH levels or ratios can still lead to die-offs.

If your plants, invertebrates, or fish are struggling despite perfect pH, stable temperature, clean water (0 ammonia, 0 nitrite, low nitrates), and appropriate salinity—your next troubleshooting step should be checking GH and the calcium-to-magnesium ratio.

Fortunately, adjusting water hardness is relatively simple:

• To lower GH: Use RODI (Reverse Osmosis Deionized) water for partial water changes. This dilutes mineral content and brings GH down.

• To raise GH: Add calcium chloride or magnesium chloride. Both are aquarium-safe, pH-neutral, and cost-effective.

Since GH is measured in ppm or degrees (°GH), a small dose goes a long way. To increase GH by 1° (18 ppm), you only need about 18 mg per liter, or approximately ⅛ teaspoon per 10 gallons.

Key Takeaway:

Understanding your water’s GH—and maintaining the proper calcium and magnesium balance—is an essential part of keeping fish healthy and avoiding unexplained deaths or plant issues. Test your water regularly, especially when problems persist despite otherwise ideal conditions.