Water chemistry parameters

pH measures how acidic or basic water is on a 0 to 14 scale. 7 is neutral. Below 7 is acidic, above 7 is basic.

Ammonia is the first nitrogen waste produced by fish, uneaten food, and decaying matter. It is highly toxic even at low concentrations.

Nitrite is the second step in the nitrogen cycle, produced when bacteria break down ammonia. It is also toxic and should read zero in a cycled tank.

Nitrate is the end product of the nitrogen cycle. It is far less toxic than ammonia or nitrite but still needs to stay in range.

Alkalinity is the water's ability to resist pH change. In aquariums it is measured as KH or dKH. In pools it is total alkalinity (TA) in ppm.

Calcium hardness is the amount of dissolved calcium in water. Critical for reef coral and coralline algae growth, and for protecting pool plaster.

Phosphate is a nutrient that fuels algae and inhibits coral skeleton growth. Comes from food, additives, and tap water.

Oxidation Reduction Potential measures the water's ability to oxidise contaminants. A real-time measure of sanitiser effectiveness.

Salinity is the amount of dissolved salt in water, measured by specific gravity (SG) or parts per thousand (ppt).

Magnesium is the third leg of the reef chemistry triangle (with calcium and alkalinity). It keeps calcium and alkalinity from precipitating out.

Free chlorine is the active sanitiser in pool and some spa water. It kills bacteria, algae, and viruses on contact.

Total chlorine is the sum of free chlorine plus combined chlorine (chloramines, the byproduct of FC reacting with organic waste).

Cyanuric acid (stabiliser, conditioner) protects free chlorine from being destroyed by sunlight. Critical for outdoor pools.

Bromine is the preferred sanitiser for hot tubs and spas. It stays effective at higher temperatures than chlorine and produces less odour.