RODIIt appears water current is quite strong disrupting the ability for water tp properly flow into sump and in a sense stirring tank.
Tank is fairly new yet and I would rely on salt rather than dosing anything to tank as your salt mix should contain the essentials and if you are doing regular water changes, you should have fairly good readings.
I question if you are getting true readings or false readings. What test kits are you using ?
Are you using tap water from faucet or RODI water ?
Continue use of UV unit and place a filter sock or nylon stocking at your drain at sump to trap small micron particles.
Refrac for salinity
pH Pinpoint monitor
Alk Hannah check
Ca Red Sea
Mag Salifetr
Nitrate Nyos
Phosphate Hannah checker