A good place to start is with pure fresh water. Think Reverse Osmosis Systems (RO Water for Drinking) De-Ionization System (DI Water for Aquarium Use). T-off your system to a drinking water reservoir to save the cost and disposal of bottled water.
A typical RO/DI system
Video URL for how to connect and disconnect John Guest fittings:
A TDS (total dissolved solids) test of a bottled water sample – reading is 95.
For the aquarium, we’re looking for 0 or very nearly so. Note that our tap water reads close to 400 and the TDS manual says water near 500 is unsafe for human consumption. Hmm….