pH, KH and GH in an aquarium: what they mean and how to adjust them

pH, KH and GH in plain terms

• pH is acidity on a 0–14 scale, with 7 as neutral. The scale is logarithmic: pH 6 is ten times more acidic than pH 7, so even half a unit is a big move.
• KH (carbonate hardness) is the amount of carbonate and bicarbonate in the water. It soaks up acids and holds pH in place, which is why it is called the buffer.
• GH (general hardness) is mostly calcium and magnesium. It does not set pH directly, but plants draw Ca and Mg from it and shrimp need it to build a new shell.

Test kits report hardness in German degrees (dKH, dGH) or in ppm as CaCO₃. One degree is about 17.8 ppm, so 4 dKH is roughly 71 ppm.

How they affect each other

The higher the KH, the more acid the water absorbs before pH moves. With very low KH, the acids from decay and from aquasoil are enough to pull pH down quickly.

Dissolved CO₂ forms a weak carbonic acid, so it lowers pH. In a tank with injected CO₂, pH falls during the photoperiod and climbs back once the CO₂ is off. That daily drop is expected. A drop of about one full unit against a degassed sample points to roughly 30 mg/l CO₂, the top of the usual 20–30 mg/l target.

Livestock usually tolerate this daily swing because KH and dissolved minerals stay the same. The pH changes to be careful with are the ones that come with a change in hardness or from a product.

Without injection the pattern runs the other way: plants use up CO₂ during the day and pH rises a little, peaking at the end of the photoperiod and bottoming out before lights-on.

Aquasoil acts as an ion exchanger. It binds calcium and magnesium and acidifies the water, so it lowers GH, KH and pH. The effect is strongest in the first weeks and fades over time, faster if your tap water is hard.

Target ranges by tank type

• planted tank with CO₂: pH 6.4–6.9, KH 3–5 dKH, GH 4–8 dGH,
• low-tech planted tank: pH 6.5–7.5, KH 3–8 dKH, GH 4–10 dGH,
• Neocaridina shrimp tank: pH 6.5–7.8, KH 2–8 dKH, GH 6–12 dGH.

These are sensible starting targets, the same defaults SUISO uses, not hard limits. Published ranges for Neocaridina vary a lot, but sources agree that steady water matters more than hitting a particular number.

Without CO₂ and with KH near the top of the range, pH naturally sits higher, often around 7.5–7.9. That alone is no reason to intervene.

If you inject CO₂, watch the fish as well as the pH. If they gasp at the surface, cut CO₂ straight away and increase surface agitation.

How to lower pH without chemicals

KH holds pH up. As long as KH is high, any acid you add gets used up and pH bounces back. That is why lasting changes come mainly from water with a lower KH.

• CO₂ injection is the usual route in a planted tank. It lowers pH during the photoperiod and feeds the plants at the same time. Set the amount by the pH drop, never by chasing a pH number: the ceiling is a drop of about 1.0 from a degassed sample, roughly 30 mg/l. That puts you in the 6.4–6.9 range only at about KH 3–5. With harder water, reaching it would take a dangerous amount of CO₂, so lower KH first or accept a higher pH.
• Aquasoil keeps the water on the acidic side by itself at first; the effect fades with time.
• RO or distilled water mixed with tap water lowers both KH and GH roughly in proportion: a 50/50 mix gives about half the hardness. Switch over several normal water changes rather than one big one, especially with shrimp.
• Driftwood, catappa (Indian almond) leaves and peat release tannins and tint the water yellow to brown. They lower pH noticeably only in soft, low-KH water and hardly at all in hard water.

pH-down products in well-buffered water give only a temporary drop. Dosing them again and again leads to swings that stress fish and especially shrimp. If you use one anyway, go carefully, in small steps and within the manufacturer’s directions.

How to raise KH and GH

Low hardness usually comes from soft source water: RO, distilled water or rainwater, or aquasoil that has used up the KH. RO water has almost no minerals and is not suitable on its own.

• Remineralizing salts raise GH, and some raise KH too. Mix them into the change water at the manufacturer’s dose.
• Baking soda (sodium bicarbonate) raises KH only, not GH.
• Crushed coral or limestone in the filter slowly raises KH, GH and pH. It takes days to weeks and is hard to fine-tune.

About 3 g of baking soda per 100 liters (roughly 1/2 teaspoon per 25 US gallons) raises KH by 1 dKH. Weigh it, dissolve it in a cup of tank water first, raise KH by no more than 1 dKH a day, then retest KH and pH. How far pH moves depends on the CO₂ in your tank.

Raise hardness over several water changes rather than in one go. This matters even more with shrimp: with too little GH they cannot get enough calcium for a new shell, and with too much the shell gets so hard that molts fail.

Common mistakes

• chasing an exact number instead of stable water,
• adding CO₂ until pH hits the target even though KH is high,
• using pH-down chemicals in water with high KH,
• changing hardness with one big water change or dose,
• refilling after a water change with water of a different hardness – pH shifts the day after the change and it looks like a problem in the tank,
• testing pH straight from the tap – dissolved CO₂ makes it read differently than it will after standing,
• stacking several methods at once so you cannot tell what moved the pH.

Top off evaporation with RO or distilled water. Only water evaporates and the minerals stay in the tank, so topping off with tap water slowly pushes hardness up.

Testing and what to log

Liquid drop tests work well for KH and GH: you count drops until the color changes, and on common kits one drop equals one degree, but check your kit’s instructions. Test pH with a liquid kit or a calibrated pH pen at the same time of day each time. With CO₂, take two readings: in the morning before the CO₂ comes on and near the end of the photoperiod.

Those two readings show the daily swing. To estimate CO₂, compare the end-of-photoperiod pH with a degassed sample (a glass of tank water left to stand for a day), not with the morning reading. Tank water still holds some CO₂ in the morning, so a drop measured from the morning value understates how much CO₂ is in the water.

A TDS meter tells you how much is dissolved, not what. It is a quick check that new water matches the tank, but it cannot replace KH and GH tests because fertilizers raise it too.

Log the date, pH, KH, GH and your water source, plus the RO ratio if you mix. When pH moves, your notes show whether the water, substrate or CO₂ changed the day before, so you are not guessing.