💧 Water chemistry

How do I lower GH in an aquarium?

How to lower GH (general hardness) in an aquarium — dilute with RO or rainwater, use peat, and reduce hardness gradually to avoid stressing your fish.

The short answer

To lower GH (general hardness) — the calcium and magnesium content — you have to dilute the minerals out, because there’s no additive that reliably removes them. Mix your hard tap water with mineral-free RO or rainwater: a 50/50 blend roughly halves GH. Peat and botanicals help a little too. Whatever you do, lower GH gradually over several water changes, since a sudden drop in hardness stresses fish more than the hardness itself.

Why lower GH

You’d lower GH for genuine soft-water species — wild-type tetras, rasboras, bettas, apistogramma, discus and caridina shrimp — which colour up and breed best in low-mineral water. If your fish are hardy community types already thriving, you probably don’t need to touch your GH at all. Match the effort to the livestock.

The reliable method: dilute

Blend RO or clean rainwater into your tap water until a liquid test kit shows your target GH, then use that mix for water changes. Test before it goes in the tank. See using RO water and is rainwater safe for how to prepare each safely.

The gentle method: peat and botanicals

Peat, catappa leaves and driftwood release organic acids that lower hardness slightly while tinting the water — great for blackwater tanks, though slower and less precise than dilution. See removing tannins if you’d rather keep the water clear.

Tip: lowering GH via RO usually lowers KH too, which weakens your pH buffer. Watch KH so pH doesn't start to swing — see what a buffer is.

For the full picture see softening water and KH and GH explained. More in the water testing hub.

Frequently asked questions

Does lowering GH also lower pH?

Not directly — GH measures minerals, not buffer. But the methods that lower GH, like adding RO water, usually lower KH too, and less KH can let pH drift. Test both and go gradually so pH doesn't swing.

Can I lower GH with chemicals?

There's no reliable additive that removes hardness minerals the way dilution does. Ion-exchange resins help temporarily, but cutting your water with RO or rainwater is the controllable, predictable method most keepers use.

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