It is impossible to have no gH change if you did a 50% RO water change. Check your test, check your measuring process. You're doing something wrong. Stones cannot increase water hardness quickly, there is a slight hardness increase over time. In days or weeks, not several hours.
If you have a TDS meter. Test the RO water with it, test the aquarium water. Compare, do another 50% change, test again.