Aquarium Water Parameters: pH, GH and KH

The six readings at a glance

The first three come from the nitrogen cycle. Our cycling guide explains where they come from and how to get them under control.

The other three describe the water itself, and they are mostly set by your tap.

pH

pH runs from 0 to 14, and 7 is neutral. Below 7 is acidic and above 7 is basic. Each whole number is a 10-fold change, so a small change in the number is a large change in the water.

• One retailer gives pH 6.5 to 8.0 as the range where most freshwater fish do well.
• pH in a tank naturally shifts during the day.
• Sudden spikes or crashes are what cause stress, shown as frantic swimming, lethargy or rapid breathing.
• A veterinary manual says fish adapt to a gradual pH decline, often with no outward sign of distress.

GH and KH

GH (general hardness) is the amount of calcium and magnesium in the water. KH (carbonate hardness, also called alkalinity) is the amount of carbonate and bicarbonate. KH neutralizes acid, which helps stop pH from swinging.

They often move together, but not always. Water can be high in one and low in the other.

Kits report these in degrees (dGH, dKH) or in ppm. One degree of GH is 17.86 mg/L, and mg/L is the same as ppm. One retailer gives one degree of KH as 17.9 ppm.

Words like “soft” and “hard” are not fixed, as these two sets of bands show:

Sixty mg/L is about 3.4 dGH, so water the USGS calls soft is “very soft” in the magazine’s bands. Go by the number, not the label.

How much GH and KH matter

Sources disagree here. One retailer gives 4 to 8 dGH as ideal and links low GH to poor appetite, slow growth and faded color. One keeper site argues GH does not matter for adult fish, and says they survive well outside their published ranges. Both agree snails and shrimp need some minerals: the retailer lists molting trouble in shrimp and thin, pitted snail shells as signs of low GH.

We show each species’ sourced range and treat it as a guide, especially for breeding and for sensitive animals.

Low KH is worth watching

Water without enough KH can let pH fluctuate dangerously. One retailer warns at 2 dKH or below, about 36 ppm. A veterinary manual says alkalinity of 50 mg/L or less stops the filter’s bacteria working normally. Neither is a level to aim for, so treat a low reading as a reason to find out more.

How to test your tap water

1. Ask your water supplier

   Water utilities typically tell consumers how they disinfect the water. Most city water uses chlorine or chloramine, which is chlorine combined with ammonia. Hardness and pH vary from place to place, and well water can differ again.

2. Get a test kit, or borrow one

   One trade association lists ammonia, nitrite, nitrate, pH and hardness as the tests to run. Add KH, because it explains pH swings. A veterinarian or pet store may test a sample for you. Having your own kit lets you test at every water change.

3. Test the water straight from the tap

   Source water can contain ammonia. Note any nitrate as well, because a water change cannot lower nitrate below what your tap adds.

4. Check how your ammonia kit works

   One of the two common kit types, the Nessler method, can read falsely high after some water conditioners. The salicylate type is not affected.

5. Write the numbers down

   Your tap numbers are the baseline for everything else, including which species to look at. Keep them with the date.

Then keep testing the tank. Sources range from at least weekly to once every two weeks. A university guide says to test immediately whenever fish look or act differently.

How to read the results

A welfare charity gives zero as the safe level for both ammonia and nitrite. If either reads above it, the charity says to change half of the water every day and keep testing until conditions improve. Those figures are for a tank in trouble, not a routine.

Match your water to your fish

This is where your tap numbers pay off. Open our fish list with your closest match already filled in:

1. Pick your pH and GH

   Use the closest option to your tap numbers. You can set both on the fish list, along with your tank size.

2. Open a species page

   Each one shows the exact sourced range. Check that your number sits inside it, not just near it.

3. Check shrimp and snails separately

   Our invertebrate list has the same filters. A card says so when a figure is not published, so an unknown is never mistaken for a match.

4. Prefer your water over changing it

   If your tap suits a good range of species, start there. If you do change it, one retailer advises gentle methods, because sudden swings are what stress fish.

If your water is hard, our hard water list is a good starting point. Once you know what suits your water, choose a tank size.