updated Apr 2021
The water that goes into our tank is largely 'invisible' (we tend not to think too much about it) but it can have a huge impact on plant and livestock health.
TDS: Total Dissolved Solids
TDS measures all dissolved organic & inorganic substances in the water. What makes up the TDS value matters infinitely more important than the value itself. 100pm of Calcium in water is relatively harmless, 3ppm of copper will kill most aquatic life. Dosing fertilisers in a planted tank will naturally raise the TDS value; in their simple elemental forms most fertilisers are non-toxic to livestock unless over-dosed greatly. The exception are terrestrial root tabs that contains large amounts of ammonia (e.g. osmocote+) or copper.
GH: General Hardness
Despite its fancy name, GH just measures the amount of Ca/Mg ions in the water (and other divalent cations). Calcium is present in most tap water. However, magnesium is often over-looked. Most plants are tolerant over a wide range of GH unlike KH. Important to have about 4dGH if keeping shrimp.
KH: Carbonate Hardness
KH measures Carbonate hardness/alkalinity. Essentially it measures the water's buffering capacity; the higher the KH, the higher the pH in absence of other chemicals in the water, and the more resistant the water is to downward fluctuations when an acid is added. Pure water with 0 KH will have a pH of 7. Affects fish/plant osmoregulation and this variable should be kept stable.
When people say that some plants prefer softwater, it actually refers to low KH/low alkalinity water, not low GH water per se. Picky species can be kept fine if the KH was low, but GH high. GH and KH can be adjusted/influenced separately, though the common compound that affects both at the same time; limestone (CaCO3) is what most commonly causes hardwater (limestone causes an increase in both GH and KH at the same time). Raising the GH without raising the KH can be done using calcium/magnesium sulphate; CaSO4 and MgSO4. Raising KH without raising GH can be done using potassium carbonate K2CO3.
pH measures how acidic or alkaline the water is. pH fluctuations from CO2 does not harm fish (even though high levels of CO2 can). This is because CO2 is not a salt, and it is the changes in salt concentrations in water that impact osmotic functions of livestock. pH fluctuations because of KH flux can kill livestock; this is not connected to the change in acidity, but change in salt concentrations that impact osmotic functions.
However, extreme values of pH (high or low) can affect livestock. A normal range of value would be between a pH of 6.0 to 8.0. For most tanks pH values do not fluctuate to a point of being harmful as long as the KH is kept stable.
Livestock from regions of higher or lower pH levels can be more suited to the outliers on this range i.e. fish from acid peat swamps may do well in pH as low as 4.0, while many cichlids from alkaline water lakes are comfortable in higher pH ranges. If keeping fishes that are less tolerant to low pH; buffering the water to have higher KH levels (3 dKH), will prevent pH levels from dropping too low due to CO2 injection. For most tanks, having 2-3dKH of alkalinity in more than adequate to prevent water from becoming overly acidic such that it affects livestock.
For further details on pH, KH, GH and TDS, explore the articles here.
Above: plants such as Centrolepis Drummondiana 'Blood Vomit' and the red Eriocaulon quinquangulare prefer very softwater.
AMMONIA TEST KITS
TESTING GH / GENERAL HARDNESS
TESTING FOR NITRATES
TESTING FOR PHOSPHATES
TESTING FOR TDS / TOTAL DISSOLVED SOLIDS
For increasing Magnesium levels, use a magnesium sulphate supplement such as EPSOAK MAGNESIUM SULPHATE below. Plants need magnesium to grow well and magnesium may not be present in tap water. Oddly, it is also missing in many commercial fertilizer mixes. Over-dosing may stunt some plants (>10ppm), as with most fertilizers, a little goes a long way.
SEACHEM EQUILIBRIUM raises GH (Both Ca and Mg) without raising KH. This may be useful situationally in very softwater. If your tap water has less than 1dGH, this could be useful to raise water hardness. It also contains a good amount of potassium, so dosing with additional potassium may be unnecessary if using this to increase GH.
CALCIUM SULPHATE (CaSO4) raises Calcium levels in tanks with very softwater (<1dGH). Has no impact on KH. This allows one to raise GH levels while keeping alkalinity (KH) low.
For increasing KH levels without increasing GH, use POTASSIUM BICARBONATE or the NILOCG KH BOOSTER. Increasing carbonate hardness/alkalinity increases the pH of the water. This is done more for livestock than plants as most plants grow better in soft water rather than hard water. (Exceptions being Pogostemon helferi, Valisneria species).