Raise an aquarium’s pH: check the KH first, then adjust it without stressing the fish

The test reads 6.0, while your guppies, platies, or African cichlids prefer neutral to alkaline water. A common reaction is to pour in “pH Up” and repeat when the value drops again. To raise the pH of an aquarium sustainably, you almost always need to act on the KH first, that is, on the reserve of carbonates and bicarbonates that stabilizes the water, and then make slow adjustments, no more than a few tenths per day. Without sufficient KH, the pH rises for an hour and drops again the following night. Here is how to diagnose the cause and choose the method suited to your tank without putting your fish at risk.

pH and KH: the pair to understand before taking action

pH measures the acidity of water on a logarithmic scale: going from 6 to 7 means dividing the concentration of acidic ions by ten. KH, or carbonate hardness, measures the amount of dissolved bicarbonates and carbonates. These act as a buffer: they neutralize the acids continuously produced in the aquarium. As long as the KH is adequate, the pH remains stable. When it is depleted, the pH becomes unstable and eventually drops, sometimes abruptly.

In an aquarium, sources of acidity are constant. Nitrification, meaning the transformation of ammonium into nitrates by filter bacteria, consumes alkalinity. The respiration of fish and bacteria produces CO2, which acidifies the water. Wood, catappa leaves, and peat release organic acids. Without water changes, the KH therefore decreases month after month, and the pH follows.

Before correcting: the ammonia trap

This is the point most often overlooked, and it can be costly. In acidic water, the toxic nitrogen excreted by fish is found mainly as ammonium ions, which are relatively harmless. As the pH rises, an increasing proportion becomes free ammonia, which is much more toxic to the gills. In a neglected aquarium whose pH has collapsed, ammonium may have accumulated without visible harm; a rapid rise in pH then turns it into poison. If the pH has fallen very low in a poorly maintained aquarium, measure ammonium before making any correction and start with partial water changes rather than a product.

Method 1: water changes

This is the gentlest and often the most effective method. If your tap water is harder and more alkaline than the aquarium water, each partial water change restores KH and dilutes accumulated acids. Make moderate changes of around 10 to 20% of the volume, repeated every two or three days, rather than a massive change that would cause all the parameters to vary abruptly. Use a water conditioner if your tap water is chlorinated. In Belgium, as in the rest of the Union, drinking water must have a pH between 6.5 and 9.5 according to the directive on water intended for human consumption, but its hardness varies greatly from one municipality to another: measure it.

If you use reverse-osmosis water or rainwater, it is often the reason for a KH close to zero. These waters are practically free of minerals. They must be remineralized or mixed with tap water according to the needs of the species being kept.

Method 2: aeration and CO2

Dissolved CO2 forms carbonic acid. A heavily stocked aquarium, poor surface agitation, or improperly adjusted CO2 injection can result in a low pH despite a correct KH. In this case, an air stone, a filter outlet positioned to agitate the surface, or reducing the CO2 flow is often enough to raise the pH by a few tenths. In a planted aquarium with CO2 injection, turn off or reduce the CO2 at night, when the plants do not use it.

Method 3: Calcareous substrate and rocks

For species that require hard, alkaline water, such as cichlids from Lake Malawi and Lake Tanganyika, part of the substrate made of coral sand or aragonite, or limestone rocks in the décor, slowly release carbonates. The effect is gradual and long-lasting, making this a convenient solution. However, it is not a good idea in a tank intended for soft, acidic-water fish: tetras, angelfish, discus, or corydoras would not thrive there. A small bag of crushed coral placed in the filter lets you control the effect and remove it easily if the KH rises too much.

Method 4: commercial buffers and bicarbonate

Products such as pH/KH Plus directly add carbonates and bicarbonates. They act quickly and are easy to dose, provided you follow the manufacturer's instructions and proceed in small steps. Food-grade sodium bicarbonate, pure and without additives, works on the same principle. To put this into perspective, a simple chemical calculation shows that one gram of sodium bicarbonate dissolved in 10 liters increases KH by approximately 3 °dKH. That is a lot: in a 100-liter tank, a few grams are enough to noticeably alter the balance. Dissolve the powder in a little aquarium water, pour it slowly into a well-circulated area, then wait a few hours before measuring again. Bicarbonate adds sodium: it is suitable for a one-off correction, but for regular use, products formulated for aquarium use or a suitable substrate are more balanced.

What pH should you aim for based on your fish?

These ranges are approximate. Stability often matters more than the exact value: a fish acclimated to a stable pH of 7.0 will fare better than a fish subjected to fluctuations between 6.2 and 7.6.

How quickly should you correct it?

Slowly. A variation of a few tenths of a pH unit per day is generally well tolerated; beyond that, fish may show signs of stress: rapid breathing, jerky swimming, folded fins, loss of color. For a difference of one unit or more, spread the correction over several days. Measure every day at the same time, ideally in the morning, when the pH is at its lowest after the night.

The most common mistakes

Correcting the pH without knowing the KH. A “pH up” product poured into water without buffering capacity gives a correct reading for a few hours, then everything drops again. You repeat the process, the reading yo-yos, and the fish experience far more stress than with a low but stable pH.

Relying on a single test. Test strips are convenient for quick screening, but for making adjustments, a drop test is more accurate. If in doubt, check with a second test or have a sample tested at a store.

Measuring at any time. In a planted aquarium, the pH rises during the day, when plants consume CO2, and drops again at night. An evening measurement can hide a much lower morning pH. Always measure at the same time.

Using multiple methods at the same time. A water change, a bag of coral in the filter, an air stone, and liquid buffer on the same day: the combined effect is unpredictable. Implement one measure, observe for two or three days, then adjust.

Treat the cause so it doesn't happen again

• Maintenance: regular water changes replenish the KH consumed by nitrification.
• Stocking: an overstocked tank quickly acidifies its water. Adjust the number of fish to the tank volume.
• Decor: a lot of wood, leaves, or peat in a freshwater tank with low buffering capacity lowers the pH; use them according to the species.
• Filter: a very clogged filter medium accumulates organic matter. Rinse it in aquarium water, never under the tap.
• CO2: adjust the injection according to the KH and lighting.

If you're just getting started with testing, our article how to test the pH of water explains the different measurement methods. For the opposite approach, when the water is too alkaline for your species, see how to lower the pH of an aquarium. And to organize your water changes, when to change the water in an aquarium provides a practical schedule.

Also read

For more information about water hardness: GH and KH in aquarium water.

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Frequently asked questions

— Why does the pH of my aquarium keep dropping?

Most often, KH is depleted: nitrification and CO2 consume the carbonate reserve. More regular water changes, or adding carbonates, stabilizes the situation.

— Can baking soda increase the pH of an aquarium?

Yes, by raising KH. One gram in 10 liters increases KH by approximately 3 °dKH: proceed with very small doses dissolved beforehand, and measure before adding more.

— How much can you increase pH per day?

No more than a few tenths per day. For a difference of one unit or more, spread the correction over several days, measuring every morning.

— Is it dangerous to increase pH quickly?

Yes, especially if ammonium has accumulated: in a more alkaline environment it turns into toxic ammonia. Measure ammonium and start with partial water changes.

In summary

• KH stabilizes pH: measure it before making any correction.
• Water changes, aeration, calcareous substrate or carbonate buffer raise the pH depending on the cause.
• Correct by no more than a few tenths per day, measuring every morning.
• Beware of ammonium in a neglected tank: a sudden increase in pH makes it toxic.
• Choose the target according to the species and prioritize stability.

Last updated: October 3, 2026.