Testing water: pH, hardness, chlorine and nitrate

A test strip is a plastic stick with small pads. Each pad holds a chemical that changes colour depending on how much of one substance is in the water. You dip the strip, wait the stated time, and match each pad to a colour chart. Real drinking-water labs use more precise instruments, but strips are the idea in miniature, and aquarium keepers use them every week.

pH tells you how acidic or basic water is. The scale runs from 0 to 14, with 7 neutral. It is a logarithmic scale: each step is a factor of 10 in the amount of hydrogen ions. Water at pH 6 has 10 times more hydrogen ions than water at pH 7, and pH 5 has 100 times more. (The Chemistry Lab explains why.) For drinking water the WHO gives no health-based limit for pH, but suppliers aim for roughly 6.5 to 8.5, because water outside that range corrodes pipes (and so can pick up metals) or leaves deposits, and tastes odd.

Hardness measures dissolved calcium and magnesium. Hard water forms limescale in kettles and makes soap lather poorly. It is not a health risk. Aquarium keepers split it in two: GH (general hardness, all the calcium and magnesium) and KH (carbonate hardness, which buffers the pH so it does not swing).

Chlorine is added to tap water in many countries to keep it free of germs on its journey to your tap. The WHO guideline value for free chlorine is 5 mg/L. Real tap water holds far less, usually below 1 mg/L, and a swimming-pool smell tells you the supplier is being cautious rather than sloppy.

Nitrate comes from fertiliser, manure and sewage. In drinking water the main worry is for babies under about six months, whose blood can carry less oxygen if they drink water with a lot of nitrate (a condition sometimes called blue-baby syndrome). The WHO guideline value is 50 mg/L of nitrate (NO₃⁻).

A units trap. Nitrate can be reported two ways. As nitrate (NO₃⁻) the whole ion is counted. As nitrate-nitrogen (NO₃-N) only the nitrogen atom in it is counted. Nitrogen makes up 14 of the ion's 62 units of mass, so a result in NO₃-N is 4.43 times smaller than the same water reported as NO₃⁻. The United States limit is 10 mg/L as nitrogen, which is about 44 mg/L as nitrate, close to the WHO's 50 mg/L as nitrate. Always check which unit a number is in.

Ammonia and nitrite are different. In drinking water they should be absent or very low, because they usually point to fresh sewage or farm waste. In an aquarium they matter even more: both are toxic to fish even at small amounts, so keepers aim for zero of each, and for nitrate below about 40 ppm (many act well before 80). Tap water needs treating to remove chlorine and chloramine before it goes into a tank, because they harm fish gills. A fish tank and a drinking-water supply have different targets, so the same number can be fine for one and wrong for the other. (This is general information about water chemistry, not a care guide for any species.)