Testing Water Hardness ... TDS Meter?

According to my son when he worked for a water testing company, GH measures only divalent cations. In drinking water, these comprise almost exclusively calcium and magnesium ions. There are only trace amounts of other divalent cations. So GH testers measure only calcium and magnesium ions.

But drinking water also contains monovalent cations, anions of all valencies and non-ionic compounds. TDS measures all these as well - it stands for total dissolved solids. TDS meters are a better way of measuring the 'softening' of water with RO because fish that like soft water like all those other things to be low as well.


GH testers will do the job, but TDS meters are better for this purpose, just more expensive. People who breed uncommon apistogrammas would use nothing else as their tank water has to be perfect.

To be honest, I've never looked at the prices of TDS meters. From reading on other forums I had the impression they were much dearer than that!

Edit:
Quick look on-line and I see you can pay well over £50 for some.

It's mainly the cichlid forums that talk about TDS meters. Some of the south American cichlids need very soft, acid water to breed and those who want to specialise in breeding rare apistos, for example, would probably go for the most expensive equipment they can find 
For general purpose use, the one you linked to should be fine.