It's always wise to test so called 'RO' water. I tested the water a LFS claimed was RO and it turned out to be filtered tap water. PH over 8! I found a reliable RO machine I use now but still will do a quick taste test every time to make sure. How is it your aquarium water is so much harder than the tap water? Could your gauge be inaccurate?Got a TDS meter for my CPs. Aquarium tests out just over 300 ppm. Tap is 195...RO water from my LFS? 210!
Not very happy about that...