Hi welcome to the forum
Don’t do anything to quickly as in that size tank a small change will have a dramatic effect.
The best thing to do is ditch the hydrometer and get a D&D refractometer and then recheck it.
I would take out 100ml of tank water and replace it with 100ml of plain ro.
But as I say get a refractometer or get it checked by your lfs or a friend first.