To raise GH, add calcium and magnesium in a controlled way. Aquarium-specific remineralisation salts are the most predictable option, especially when using RO water. Mineral-rich media such as crushed coral can also raise hardness gradually, but they may raise KH and pH at the same time. Make changes slowly and prepare replacement water to the same target so the tank does not swing at every water change.
Low GH is not just a number. It means the water contains relatively little calcium and magnesium, which can matter for fish physiology, shrimp moulting, snail shells and plant growth. The safest correction is controlled remineralisation rather than repeatedly adding mystery powders to the display tank.
Confirm GH is genuinely too low
Test the source water and aquarium with the same kit. Compare the result with the needs of the species you keep rather than aiming for a universal hardness number.
If the livestock already suits soft water and the aquarium is stable, there may be nothing to fix.
Use a dedicated remineraliser for precision
Products designed to raise GH supply calcium and magnesium in known proportions. They are particularly useful when starting with RO or deionised water because the base water contains very little mineral content.
Dissolve and measure outside the aquarium where possible, then add replacement water gradually.
Know what crushed coral changes
Crushed coral dissolves more readily in acidic water and can increase GH, KH and pH together. That can be useful in very soft water but is not ideal when you want to change GH without strongly altering buffering.
Use small amounts, monitor the trend and remember that the effect is gradual rather than instant.
Do not raise hardness in one jump
- Make modest changes over successive water changes.
- Keep temperature matched.
- Retest GH after the water has mixed fully.
- Watch KH and pH because some mineral sources affect them too.
- Keep the final replacement-water recipe repeatable.
Stability beats constant correction
Once the aquarium reaches a suitable range, stop chasing the number. The long-term goal is stable water that broadly matches the animals' needs, not constant chemical adjustment.
Related aquarium tools and guides
Frequently asked questions
Can crushed coral raise GH?
Yes. It can add calcium carbonate and commonly raises GH, KH and pH together.
Can Epsom salt raise GH?
Magnesium sulfate raises magnesium hardness, but using one mineral alone can distort the calcium-to-magnesium balance. A complete aquarium remineraliser is usually easier to control.
Should I raise GH directly in the tank?
Preparing replacement water outside the tank gives better control and reduces sudden changes.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.