Why General Hardness (GH) Matters for Your Aquarium Fish

When setting up an aquarium, most hobbyists obsess over pH and temperature. But there’s another, often overlooked water parameter that quietly governs your fish’s health: General Hardness (GH).

If you’ve ever lost fish “for no reason,” or watched them appear healthy one day and die the next, GH could be the hidden culprit.

What is GH?

GH measures the amount of calcium (Ca²⁺) and magnesium (Mg²⁺) in your water. These aren’t just numbers on a test strip they are vital minerals that fish absorb through their gills. Calcium and magnesium play key roles in:

• Heart function and muscle contractions

• Nerve impulses

• Scale and fin development

• Enzyme and hormone function

Think of GH as the micronutrient backbone of freshwater fish physiology.

Why Low GH is Dangerous

Fish live in water that’s less salty than their own bodies. This means they are constantly losing salts while water flows in a delicate balance called osmoregulation.

At very low GH (around 1 dGH, which is extremely soft water), there’s almost no calcium or magnesium available. Fish must work overtime to maintain their internal balance, leading to:

• Chronic stress

• Weakened immunity

• Sudden deaths with no visible symptoms

This is why hobbyists often see “perfect-looking fish” die overnight — it’s a mineral deficiency at work.

Calcium and Magnesium: The Stress Shield

Calcium stabilizes gill membranes and helps fish tolerate:

• Netting or handling

• Water changes

• Temperature fluctuations

• Salt treatments

Magnesium supports muscle function and general physiological stability. Without enough of these minerals, even small environmental stresses can become fatal.

Species Sensitivity to GH

Not all fish are equally affected. Some tolerate soft water, but most prefer at least moderate GH levels:

Fish Group	Sensitivity to Low GH
Tetras	Moderate
Corydoras	High
Discus	Moderate, needs stability
Livebearers (Guppies, Mollies)	Very high
African cichlids	Extremely high

Even “soft water species” rarely experience GH <1 dGH in nature for long periods. Stability is more important than absolute softness.

GH, KH, and pH: The Balancing Act

• KH (carbonate hardness) buffers pH, keeping it from swinging wildly.

• GH does not directly control pH, but low GH makes fish more sensitive to pH changes.

Once GH is corrected, your fish tolerate normal pH fluctuations much better you won’t need to constantly chase pH numbers.

GH and TDS

Total Dissolved Solids (TDS) measures everything dissolved in water, including minerals. Raising GH increases TDS but in a good way. Fish thrive on mineral-based TDS rather than empty water or sodium-based salts.

A GH of 6–8 dGH provides a safe, forgiving environment for most ornamental species.

Correcting Low GH: What You’ll See

Once GH is stabilised around 6 dGH:

• Fewer overnight deaths

• Fish show improved colour and appetite

• Reduced stress during water changes and transport

• Better acclimation for new arrivals

This isn’t luck — it’s physiology stabilising, allowing your fish to thrive.

Retail and Hobby Best Practice

• GH: 5–8 dGH

• KH: 3–5 dKH

• Use stable, consistent water for all tanks

• Avoid chasing pH artificially once GH/KH are correct

For beginners: a stable, moderately hard water is much safer than extremely soft water that looks “natural.”

Takeaway

GH is one of the most important but underappreciated parameters in freshwater aquariums. Soft water is not inherently bad, but unstable or extremely low GH leads to stress, sudden deaths, and poor long-term fish health.

By understanding and monitoring GH, even beginners can prevent mysterious losses, boost fish vitality, and enjoy a thriving aquarium.