I always misunderstood GH. I thought it was a measure of everything dissolved in the water, including carbonate, but it isn't. This is why it's possible to have high KH and low GH. It's total dissolved solids (TDS) that measures everything.
But generally speaking, if you have hard water (high GH) you will also have have high-ish KH.
That's exactly what I thought too. All the reading I'd done told me this.... until late last night on
this web site. I was looking to see about keeping a single Honey Gourami in my tank and I really rate this site as a source of reliable information, but it takes ages to wade through and find.
I found this info:
Ah now, aquarium salt -- sodium chloride -- will neither raise nor stabilise GH or KH. General hardness is primarily magnesium and calcium salts, while carbonate hardness is primarily carbonate and bicarbonate salts. Sodium chloride falls into neither category.As Sue's son says, Magnesium and Calcium are divalent positive ions (2+ elecrical charge); Mg
2+ and Ca
2+ for GH
...and Carbonate and Bicarbonate are negatively charged ions; CO
32- and (HCO
3)
- for KH
As Sue says, GH and KH are generally found together. Here in the south of England it's very chalky and a hard water area. Chalk is Calcium Carbonate; CaCO
3, so when this dissolves in water it splits into Ca
2+ which will raise your GH, and CO
32- which will raise your KH.
When Sue added BiCarbonate of Soda (Sodium Hydrogen Carbonate; NaHCO
3) to her water, this split into Sodium ions, monovalent positive, Na
+, so this
didn't add to the GH; and BiCarbonate ions, (HCO
3)
-, which
did add to the KH.
However, the added Na
+ does add to the Total Dissolved Solids, TDS, and this will affect the osmotic pressure on your fish (again from the linked site). This is why water softeners are not a source of soft water suitable for fish. Water softeners (either cartridge kitchen ones or plumbed in salt ones) replace the Ca
2+ with Na
+, so the water isn't classed as 'hard' and doesn't fur-up your pipes, but it doesn't change the TDS or the ammount of osmotic pressure on your fish. Only Reverse Osmosis (RO) water physically removes TDS from your water.
All this is a way of saying..... sorry for the bum info Steve
, but I've learnt quite a bit from my research and combining it with Sue's post and I hope it's been helpful.