The short answer
To make aquarium water harder, add a mineral source that dissolves calcium and carbonates into the water: crushed coral, aragonite sand, limestone or a commercial GH/KH booster. Crushed coral in the filter or substrate is the classic slow-release method — it raises both GH (minerals) and KH (the pH buffer) and self-limits as pH rises. As always, raise hardness gradually and test as you go rather than dumping it all in at once.
Why harden water
Hard water suits livebearers (guppies, mollies, platies, swordtails), most shrimp and snails, African rift-lake cichlids and Central American species. These animals use calcium and magnesium for shells, moulting, bones and colour, and they’re stressed by very soft, acidic conditions. Harder water also carries more KH, so your pH stays steadier — a real bonus for beginners fighting pH swings.
Slow-release methods (set and forget)
- Crushed coral / aragonite — a handful in a filter media bag or mixed into the substrate slowly dissolves, raising GH and KH. Cheap, forgiving and self-regulating.
- Limestone or Texas holey rock — hardscape that leaches minerals while looking good.
- Wonder Shell / mineral blocks — dissolve steadily and add trace minerals shrimp love.
Dosing methods (precise control)
Powdered GH+/KH+ remineralisers let you mix exact hardness into RO or soft water — ideal if you keep shrimp or want repeatable results. Test the water before it goes in the tank with a liquid test kit.
See the opposite process in softening water, the meaning of the numbers in KH and GH explained, and more in the water testing hub.