Measuring water hardness—HELP!

[Gershom]

I am using an RO filter to generate soft water for Satanoperca and discus. I have been mixing a percentage of hard water with it, to avoid getting progressively softer, and ultimately too soft.

I don’t like test strips because I have trouble deciding what color it matches, so I have been using an inexpensive meter which actually measures conductivity and converts it to “hardness”. But I noticed that between water changes, the measured hardness in the tanks increases.

Does this mean I don’t need to add some hard water, or is that interval hardness increase false (dissolved solids which don’t truly contribute to hardness)??

And the related question: is there another way to measure hardness which avoids the problem, if it is a problem?

 

[Conchonius]

TDS creep should be a natural consequence of evaporation. Suppose that your water is a uniform 100 ppm, 10% of it evaporates each week, and you do a 50% water change, topping off your tank in the process. During your first water change your TDS would go up to around 105 ppm (50% of 100/90 = 111 TDS water due to evaporation, 50% of 100 TDS water), during the second week it'd be around 108 ppm, during the third 110 ppm and so on, since the solids in the water don't evaporate and keep concentrating the solution.

Eventually this will taper off due to diminishing returns, I'd guess the asymptote is around 110%-120% of your initial hardness for 50% water changes. If you truly want to avoid it, you can make a mark on the tank to see how much water evaporates each week, top off that with pure RO water, and change the rest of the water the regular way. In effect this is similar to the auto top-off units reefers use, since water salinity matters a lot more to them.

As for alternate methods, there are liquid tests that let you calculate the hardness by titration, these are more accurate than strips and might be easier to read since they're binary. Instead of a gradient in color, there's a sudden change once the reagent can no longer buffer the material in your water. The quicker this happens, the higher your hardness is. API has a "GH/KH test kit", for example.

 

[duanes]

Agree with the above.
Each time you add the hard water, calcium is building up, and with evaporation (that allows more minerals to remain) between water changes.
I don't know if you dou do 1 water change per week, that could allow a more conscentrated build up,....
or several water changes per week that would create more reduction of mineral concentration.
I prefer to do multiple smaller water changes per week to maintain equilibrium.

Maybe adding less hard water each time doing an accurate titration could allow better deterination of that build up over time.

As a chemist I used to do titrations to determine water hardness, but that was in a lab, set up with multiple burettes and reagents,(a bit fidgety for most hobbyists) and even then in the lab, one tiration drop too fast, to the next coulld cause a do over.
That's why those meters help.
In the end I liked the TDS/conductity meters.