How to lower pH in an aquarium (and how to raise it)

AquaLog guide · Published 6 October 2026

Do you need to change your pH at all?

Often not. Most freshwater fish cope with a range, and sudden swings stress them more than a steady value a little outside their ideal. Neon tetras (pH 5.5–7.5) and zebra danios (pH 6–8) both span two full pH units. Check your species' range in the care guide, then test your tap water. If the two overlap, leave the pH alone.

If they don't, the easiest fix is choosing fish that suit your water: see fish for hard, alkaline water and fish for soft, acidic water. Soft-water fish such as cardinal tetras (pH 4.5–7) or discus (pH 5.5–7) in hard tap water are the cases where changing your water is worth the effort.

Why does KH decide your pH?

KH, or carbonate hardness, is the buffer. Carbonate and bicarbonate neutralise acid, so in water with plenty of KH it takes much more acid to lower the pH. That is why leaves and peat barely move the pH of hard tap water. In water with little KH the opposite happens: pH swings easily and can crash. To move pH and have it stay moved, change the KH. KH vs GH in an aquarium explains both.

Why not just use pH-down or pH-up chemicals?

Because they work against the buffer instead of changing it. pH-down products are acids, and KH is a measure of how much acid the water can absorb before its pH falls. In well-buffered water it takes a lot of acid to move the pH, and every dose uses up some of the KH. As KH falls, the water loses its buffer, so pH swings more easily and can drop suddenly. Then the next water change brings in fresh tap water with its old KH and pH. Lowering KH itself, by blending in RO water, gives a steadier result.

Raising pH quickly has its own danger. Ammonia is far more toxic at high pH, so a tank with any ammonia in it can turn dangerous when pH jumps. If you do use a pH product, follow the label, make small changes, and test KH as well as pH.

How do you lower pH safely?

1. Test pH, KH and GH in both your tap water and the tank, so you know what you are working against.
2. Mix in RO water. Reverse-osmosis units strip out most dissolved minerals. Blending RO with tap water lowers KH, GH and pH together, and a 50:50 mix roughly halves the hardness. Mix it in a bucket ahead of time, test it, and use the same blend at every water change. To size each change, enter the tank's KH, your target and the blend's KH (all in the same unit) in the water change calculator; it also shows how to spread the drop over two or three changes.
3. Never use RO water on its own. Fish in mineral-free water can die suddenly. Blend it with tap water or add a remineralising salt.
4. Add tannins for a gentle extra push. Leaf litter such as Indian almond (catappa), oak or beech leaves, alder cones and peat release tannins and humic substances that stain the water tea-coloured and lower pH slightly and gradually. Aquarium driftwood also releases tannins and can lower pH slightly in soft water. In hard water the buffer neutralises these acids, so they barely work. Use leaves that have fallen naturally, from trees away from roads, and avoid any species known to be poisonous.
5. Go slowly. Shift pH a little at each water change, over days or weeks, not hours.

How do you raise pH?

• Crushed coral or aragonite in a filter bag or mixed into the substrate. Both are mostly calcium carbonate, so they raise KH, GH and pH together. Calcium carbonate dissolves in acidic water, and limestone stops dissolving at around pH 8.3, so the effect levels off. This is the usual choice for hard-water fish such as mollies (pH 7.5–8.5).
• Baking soda (sodium bicarbonate) raises KH without adding GH. Dissolve a small, measured amount in tank water, add it, and re-test before adding more.
• More surface movement. Carbon dioxide builds up in heavily stocked or poorly aerated tanks and lowers pH. Better aeration lets it escape.
• Remove what is lowering it. Take out peat or a heavy load of leaves, and keep up regular water changes.

Why does my pH keep dropping?

Usually because KH is running out. Filter bacteria release acid as they process waste and use up carbonate as they go. In soft water, or when water changes are infrequent, KH slowly runs down until pH falls fast and the bacteria themselves stop working, so ammonia rises too. Rotting food and plants, heavy stocking, and CO2 injection in a planted tank all add more carbon dioxide or acid. Test KH, step up your water changes, and restore KH if it is near zero.

Which method should you use?

Track pH and KH together. AquaLog keeps both on a chart per tank, so you can see a slow slide coming before it turns into a crash. Start logging your water. For what the other readings mean, see aquarium water parameters explained.