Tank pressure is based on what water depth you measure it at (deeper=higher), and little else affects this- TDS will, but only to a negligable extent.
If any tank is commercially available, it will hold any volume of water (of any quality) you can fit in it without risk, unless you fill it with motor oil or something.
My question is, why would you even be concerned or bother with measuring this aspect?
The pressure of a CO2 tank that isn't nearly empty depends only on the temperature of your room. If you want to know how much is in the tank, you have to weigh it and substract the weight of the empty tank. The pressure will only go down shortly before the tank is completely empty.