Source water
The most efficient decision in the hobby is to keep species suited to your source water rather than continuously correcting the water to suit the species. Suppliers publish hardness and nitrate figures for each supply zone, and those figures do more to determine what will thrive than any product on a shop shelf.
In one sentence
Everything in the tank starts as whatever comes out of the tap. Knowing your supply's hardness, pH, nitrate and disinfectant is the starting point for deciding what you can keep easily.
What to find out
- Hardness, in whichever unit the supplier uses — convert to °dGH for comparison with fish requirements.
- Whether the supply uses chlorine or chloramine.
- Nitrate concentration, which in some agricultural regions is high enough to matter.
- Whether the supply is blended or switched seasonally between sources, which changes hardness through the year.
Well water and softened water
Private well supplies are not treated and can carry high iron, high or unstable hardness, low oxygen and occasional contamination; test before use. Domestically softened water is a specific hazard: an ion-exchange softener replaces calcium and magnesium with sodium, so it lowers GH without lowering total dissolved solids, and it is not suitable for aquarium use. Draw from an unsoftened tap, usually the kitchen cold tap.
Reverse osmosis
RO water is close to pure and must be remineralised before use — pure water has no buffer and no minerals for osmoregulation. It is used blended with tap water to reach a target hardness, which is how soft-water species are kept in hard-water regions.