There are two kinds of water hardness, permanent hardness and temporary hardness. Permanent hardness is the measure of dissolved calcium and magnesium in the water. Temporary hardness is the measure of dissolved carbonates and bicarbonates in the water, the bicarbonates needn't necessarily be of calcium or magnesium which can lead to some confusion.
A third measure is used which is overall hardness, this is a measure of both permanent hardness and temporary hardness added together.
1, Overall hardness - GH
2, Temporary hardness - KH
Only dissolved compounds of magnesium and calcium make the water hard, other dissolved solids make like salt for example make no difference to the hardness of the water. When measuring permanent hardness carbonates and bicarbonates are excluded from the measurement but all other compound of calcium and magnesium such as sulphates, chlorides ect measured.
Permanent hardness can only be changed by - dilution, not an exchange or filtration through a device like an r/o unit. Some rechargeable ion exchange resins simply replace the calcium and magnesium ions with sodium and so do not lower the TDS (total dissolved solids) count even though the hardness falls.
Temporary hardness is the measure of all the carbonates and bicarbonate in the water and not just the carbonates and bicarbonates of calcium and magnesium.
When measuring the temporary hardness you are really measuring the buffering capacity of the water and not the hardness.
Temporary hardness can be removed by boiling and it is this which is responsible for furring up the heating elements in kettles is areas with a high carbonate hardness.
Temporary hardness does affect how the pH works in the aquarium the higher the KH the less likely the pH is to fall.
0 - 50mg/l or 0 - 3dGH = Soft water
51 - 100mg/l or 3 - 6 dGH = Fairly soft water
101 - 200mg/l or 6 - 12 dGH = Slightly hard water
201 - 300mg/l or 6 - 12dGH = Moderately hard water
301 - 540mg/l or 18 - 30dGH = Hard water
541+mg/l or 30+dGH = Very hard water
Reverse osmosis units filter everything out of the water and leave virtually pure water to be collected and stored. This pure water can then either be mixed with untreated water in order to dilute the hardness to the desired level or a better option, it can be re-mineralised to the desired level.
Rain water can be used in the same way as the r/o water but it has to be collected and treated carefully in order to avoid problems.
Collect the rain water from a clean non-calcareous and non-metallic surface. Store it is a dark, well covered, clean, plastic container. Aerate before use in order to drive off any CO2. Although probably not really necessary some people prefer the added security of filtering through carbon before use. If you live in an heavily industrialised area then the r/o unit is a better option for you. If there is some light industry or busy road network near you then ignore the first 30 minutes of any rainfall before starting to collect it, especially after a prolonged dry spell. This will get rid of most of any pollution.
Is rain water safe?
Well our native fish have lived in it for thousands of years, when you think about it there is no way that the rain will get any cleaner once it has fallen!!! most ends up in a river very quickly.
Ion exchange resins can remove all kinds of impurities from water and leave just pure water but there are different kinds of resins, some are made to be easily rechargeable in a domestic situation using salt, although these will lower the hardness reading they will not alter the the TDS of the water because they replace the calcium and/or the magnesium with sodium.
Adding calcareous substrate or rockwork to the aquarium will cause the GH to slowly rise as will the addition of broken shells to the filter. Using a marine calcium additive is another good way of raising the GH.
Using epsom salts (magnesium sulphate) will raise the GH very easily but this compound has a known laxative effect on fish and so isn't recommended.
The KH is lowered in the same way as the GH but additionally boiling will remove the KH and if an acidic compound like a pH lowering product is added to the water or things such as peat or new bogwood. The KH will sacrifice itself in order to preserve the pH. This is why water with a very high KH value is resistant to lowering the pH.
If the KH is very high then use one of the methods mentioned in the lowering the GH section.
his is really very easy, simply adding some bicarbonate of soda will raise the KH level as will any aquarium pH raising compounds.
*Useful tip - 3gms/100L of bicarbonate of soda will raise the carbonate hardness by 1KH.