What is GH in aquarium water?

Why this happens

Minerals support osmoregulation, bone and shell processes, plant nutrition and many biological functions. Species evolved in very different waters, so both very low and very high hardness can be unsuitable depending on the livestock.

• Tap, rain, RO and bottled water can have very different GH.
• Evaporation and mineral-rich top-ups can raise hardness over time.
• Active soil, rock and remineralising salts may change the result.
• GH tests do not reveal every individual ion or contaminant.

A GH result is most useful alongside KH, pH, conductivity and the livestock’s origin or captive-breeding history. Stability matters, but stable unsuitable water is still unsuitable.

What to check first

• What is the GH of the source water and aquarium?
• Has the value changed between water changes?
• Do rock, substrate or additives contribute minerals?
• Are shrimp moulting, plants and fish behaviour normal?

Compare the result with the aquarium’s normal baseline, not only a generic target. Record the time, recent maintenance, new livestock, feeding changes, source-water details and any equipment fault. A short timeline often separates the original cause from secondary symptoms.

A practical action plan

1. 1. Test source and aquariumUse the same method and units so the difference is meaningful.
2. 2. Set a species-based rangeChoose a target that suits the actual stocking plan rather than a generic chart.
3. 3. Adjust prepared waterRemineralise RO water or blend sources outside the aquarium before use.
4. 4. Change graduallyAvoid large hardness shifts during routine maintenance or livestock transfer.

How to interpret the response

A consistent difference between source and aquarium suggests evaporation, substrate, rock, food or additives are changing mineral content. Track the trend before changing the recipe.

Change one important variable at a time where welfare allows, then observe and retest. When several products or adjustments are made together, it becomes difficult to know what helped, what caused stress and whether the underlying problem remains.

What to avoid

Do not add random calcium products because GH is low, assume pH indicates hardness, or make a large correction directly in a stocked aquarium without confirming units and species tolerance.

When it is urgent

Loss of balance after a major water change, repeated shrimp moulting failures, severe osmotic stress or an accidental large mineral dose requires immediate assessment and controlled correction.

How to prevent it next time

Record GH for every source-water recipe, top up evaporation appropriately, test new rock and keep the same remineralisation scale and measuring tools.

Keep enough records to recognise the aquarium’s normal behaviour, water chemistry and equipment performance. Trends are more useful than isolated numbers, and early correction is usually safer than a large emergency change.