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Problems · Care basics

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

Not urgent

Before you lower or raise the pH in your aquarium, know this: a steady pH matters far more than hitting an exact number. Most fish sold in shops do fine anywhere from about 6.5 to 8.0, and chasing a perfect figure with chemicals often causes swings that do more harm than the original pH.

The key to changing pH safely is KH, the water's buffer. Test pH and KH first. Then lower pH by mixing in reverse osmosis (RO) water, or raise it with crushed coral or a little baking soda, slowly, over days. If your pH has suddenly dropped and fish are gasping, go to the pH crash guide instead.

How to identify it

You need a test kit that measures three things. Liquid kits are more accurate than strips.

  • pH: how acidic or alkaline the water is, on a scale from 0 to 14. 7 is neutral. Below 7 is acidic ("soft" water fish like many tetras and wild plecos); above 7 is alkaline (livebearers, goldfish and African rift lake cichlids). Each step of 1 is a tenfold change, so 6.0 is ten times more acidic than 7.0.
  • KH (carbonate hardness, or alkalinity): the amount of carbonate and bicarbonate in the water. It acts like a sponge that soaks up acid and stops pH falling. High KH means pH is hard to move. Low KH means pH can swing easily.
  • GH (general hardness): the amount of calcium and magnesium. GH affects fish health, shrimp moulting and snail shells, but doesn't control pH directly.

Units: kits use either degrees (dKH, dGH) or mg/L (ppm). 1 degree is about 17.9 mg/L.

Test your tap water too, and test it again after it has sat overnight with an air stone. Tap water can hold dissolved CO2 that makes its pH read lower at first.

When it's a problem

  • Normal: pH that rises and falls a little between morning and night, especially in planted tanks. pH that is steady at any point between about 6.5 and 8.0, with healthy fish.
  • Worth changing slowly: you keep fish that need very different water to your tap (for example, African cichlids in soft, acidic water, or wild-caught blackwater fish in very hard water), or you plan to breed fish that need specific water.
  • Serious: KH near zero, with pH dropping week by week. This is often "old tank syndrome", where waste slowly uses up the buffer. The Merck Veterinary Manual notes that alkalinity of 50 mg/L (about 3 dKH) or less stops the filter's bacteria working properly.
  • Emergency: a sudden drop or jump in pH with fish gasping, lying on the bottom or dying. See the pH crash guide.

What to do right now

  1. Test pH, KH, GH, ammonia and nitrite in the tank, and pH, KH and GH in your tap water.
  2. Look up what your fish actually need. Most captive-bred fish are adapted to a wide range.
  3. If fish are healthy and pH is steady, consider leaving it alone.
  4. If KH is very low, do a water change with tap water (if your tap water has KH), to bring the buffer back up.
  5. If you have ammonia, don't raise pH yet. Ammonia becomes much more toxic as pH rises. Fix the ammonia first (see ammonia).
  6. Don't add "pH up" or "pH down" chemicals in a hurry.

What to do next

How fast is safe?

Change pH slowly. A common rule of thumb is no more than about 0.2 to 0.3 units in a day, and less for delicate fish and shrimp. The easiest way to stay safe is to make all changes through your water-change water, so the tank shifts a little each week.

How to lower pH

  • Mix in RO water. Reverse osmosis (RO) water has almost no minerals or buffer. Mixing it with tap water lowers KH, GH and pH together, and it's the most reliable method. Use the same mix at every water change. Don't use 100% RO water with nothing added: fish need some minerals, and water with no buffer can crash. Shops sell remineralising salts for RO water. Rainwater collected away from roofs and roads is another soft water source; strain it first.
  • Driftwood, catappa (Indian almond) leaves and peat. These release tannins, which tint the water tea-coloured and can lower pH a little in soft water with low KH. In hard water with high KH, they make little difference. One aquarium writer measured no change at all in high-KH water. Use them for soft-water fish and to make a gentle blackwater look, not as a way to fight hard tap water. Put peat in a mesh bag in the filter, start small, and test often.
  • Acids. Some experienced keepers lower KH with measured amounts of acid in a separate water-change container, never in the tank. This needs care, gloves, eye protection and lots of testing. It's not a beginner method.

How to raise pH

  • Crushed coral or aragonite. These are forms of calcium carbonate. Put a small bag in the filter where water flows through it. It slowly dissolves, raising KH, GH and pH, and usually settles around 7.6 to 7.9. It can't push pH much higher, which makes it safe. Replace it every few months, as it gets coated and stops working.
  • Baking soda (sodium bicarbonate). Raises KH and pH. One guide suggests about 1 teaspoon per 95 litres (25 US gallons) of new water, added with water changes, which typically gives pH 7.5 to 8.0. Add a little, wait a couple of days, test, and repeat only if needed. Don't add it directly to a tank with ammonia present.
  • Limestone or other calcium-rich rock in the tank also raises pH and hardness over time. Good for rift lake cichlids, bad for soft-water fish.

Why "pH up" and "pH down" bottles bounce back

These products push pH up or down, but they don't change the buffer underneath. In water with high KH, the buffer soaks up a "pH down" dose and the pH drifts back within a day or so. People then dose again, and the fish get a roller-coaster of swings. Many "buffer" products are mostly baking soda or phosphate. Phosphate-based buffers can also feed algae. Changing the water you start with (RO mixing, crushed coral) works better.

Cautions

  • Shrimp and snails need some GH and KH for shells. Very soft water can cause moulting problems and pitted shells.
  • Plants handle a wide range, but large swings stress them.
  • Medicines behave differently at different pH and hardness. Check labels before treating. Remove activated carbon when medicating, and finish the course.
  • Injected CO2 in planted tanks lowers pH during the day. That daily swing is normal and fish cope with it.

Products that help

  • Liquid test kit for pH, KH and GH: more accurate than strips. You can't manage pH without knowing KH.
  • Digital pH meter or continuous pH monitor: useful for planted tanks and breeders. Needs regular calibration.
  • RO unit or shop-bought RO water: the most reliable way to soften water and lower pH.
  • Remineralising salts: add back calcium, magnesium and buffer to RO water in measured amounts.
  • Crushed coral or aragonite in a mesh bag: a slow, gentle way to raise and stabilise pH.
  • Baking soda: a cheap way to raise KH, used carefully.
  • Driftwood, catappa leaves or peat: for soft-water and blackwater setups.
  • Large water-change containers: let you prepare and test water before it goes in.

How to prevent problems

  • Choose fish that suit your tap water. This is the easiest and safest option.
  • Do regular water changes so KH doesn't slowly run out.
  • Prepare water the same way every time.
  • Test pH and KH every week or two, and write the results down.
  • Don't overfeed, as waste slowly acidifies the water.

Why it happened

Your tank's pH depends mostly on your tap water and its KH. Over time, fish waste and the filter's bacteria produce acid, which uses up KH. When KH runs low, pH starts to fall and can swing. Wood, leaves and peat lower it a little; rocks, coral and shells raise it. CO2 from fish, plants and injection also lowers pH, especially at night.

When to get help

Ask for help if pH keeps falling despite water changes, if your tap water pH or KH changes suddenly (your water supplier may have changed treatment), or if fish die after a pH change. A local fish store or aquarium club often knows your local tap water well.

In the game

Multiple Tank Syndrome doesn't have a card for adjusting pH yet. The related cards are "pH crash" and the "pH monitor" that protects against it, and in the game you can only move a tank's pH by half a unit each turn. In a real tank, go slower than that.

Sources

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