I replaced about 30l of saltwater with fresh RO water to fix the SG. It is now 1.025 which seem much better. The alkalinity is taking a dip though - down from 7.5 to 6. Started adding kalkwasser again by adding 4 flat teaspoons to 13l of RO topup water. This implies I have just installed a Tunze Osmolator kit that handles auto topoff.