Following altos recommended way to change water over:
, I'm going to assume a water volume of 50 litre (assuming tank total capacity of 60l)
At present, tank is tap water with ~25GH
so if you were to do ~10% daily water changes
Day 1 remove 5l tank water & replace with RO, so 25GH x 45l/50l ~ 22.5GH
Day 2 same 5l change, so 22.5GH x 45/50 ~ 20.25 GH
Day 3 ~18.2 GH
Day 4 ~16.4 GH
Day 5 ~14.8GH
& so on, each time you are just removing 5litre tank water & replacing with 5litre RO
Day 7 ~ 12GH so easy to perform a 50% water change, refilling with a 50:50 mix of tap & RO ... or a 40:60 mix of tap & RO
At this point you can move to 20% water changes, replacing 10litre tank water with 10litre RO
At this rate, after 3 more small water changes, tank will be ~ 5GH
If this is a reasonable "target", you can now just carry on with usual water change schedule, but now refilling with your SS remineralized RO (rather than straight RO)
Everything going well I believe. As he projected my water is following nicely down wit gh/kh
I'm getting on day 4:
KH- 6
GH-16
That's a drop of approx 2units with gh and my kh has halved.