May we count your visit with Google Analytics? It uses cookies and sends visit data to Google. You can change this any time under Cookie choices. Privacy

aquarium.guide
Choose your tank

Fish pages, lists, fit marks and tools use the tank you pick here.

Plan everything for this tank →
Menu
Water tests/KH

KH in an aquarium: carbonate hardness and pH crashes

KH (carbonate hardness, or alkalinity) is the water’s buffer: it soaks up acid and keeps the pH from falling. Keep it at 100 ppm (about 5.6 dKH) or more; at 50 ppm (2.8 dKH) or less the filter bacteria struggle and the pH can crash, according to the Merck Veterinary Manual. Regular water changes restore it, and crushed coral or aragonite raises it for good.

On this page

Check all your readings →
On this page

What your kh reading means

  • Act today2.8 dKH (50 ppm) or less

    Very low: the filter bacteria struggle and the pH can drop suddenly.

  • Watch2.8 to 5.6 dKH (50 to 100 ppm)

    Low: the pH can drift down between water changes; test it more often.

  • Good5.6 dKH (100 ppm) or more

    Enough buffer to hold the pH steady between water changes.

For a tank with fish. The same bands run in the test reading checker, which reads all your results together.

What KH does

KH is the carbonate and bicarbonate in the water. Acid from fish waste and from the filter bacteria is used up neutralising it, so the pH stays where it is. As long as there is KH left, the pH barely moves; when it runs out, nothing holds the pH up.

Why it falls

Filter bacteria use carbonate as they turn ammonia into nitrite and nitrate, so KH drops steadily, faster in tanks with many or large, heavily fed fish. Driftwood, peat and leaves use some too. Water changes put it back; topping up evaporated water does not, because evaporation leaves the acid behind. Small, rare water changes are how old tank syndrome starts.

Signs of a pH crash

A pH far below its usual reading, fish breathing hard or lying still, and ammonia appearing in a tank that has always read 0: the bacteria stop working well once alkalinity falls below about 20 ppm (about 1 dKH), according to the University of Florida. Test KH and pH together, and bring them back with small daily water changes rather than one big one. Aquarium pH · Ammonia

Raising it

The steady way is crushed coral or aragonite in a filter bag or mixed into the substrate: it dissolves slowly and keeps KH topped up, and raises the pH and GH as well. More frequent water changes help if your tap water has KH. Raise it a degree or two at a time, testing as you go.

Tanks kept soft on purpose

Keepers of discus, wild tetras and other soft-water fish often run KH low on purpose. That works with frequent water changes and regular testing, because the pH has little buffer to stop it falling. With injected CO₂, KH also sets how far the pH drops: CO₂ for planted tanks.

Other water tests

Sources