I think a pH of 8.3 is just dandy
Having a higher calcium level [than NSW] is preferred by many, mainly to accelerate coral growth; I would pick up some two-part, and use an online reef dosing calculator to find out how much part two you will need to add to your system in order to reach your desired calcium level. Add the needed amount slowly to prevent any large spikes. Do the same with the alkalinity -and then after you get your calcium and alkalinity where you would like them, simply do maintenance dosing to keep things where they are. That's what I did. Others may have a different way of doing things.
I can't give much advice on raising magnesium from experience, but a handful of people just use liquid magnesium supplements (ie. Kent) to bump the levels up in their tank. With most of these supplements you need quite a bit of supplement to raise the magnesium level just a little.
PO4 -cut back on the feedings if you can. If you're overfeeding, well then ... stop
If you're feeding frozen food, try and rinse it out a little prior to feeding out. Avoid just throwing a chunk of frozen fish food in the tank and letting it thaw in there. Set your protein skimmer to skim a little more wet than dry. Consider using a PhosBan reactor (or any other similar fluidized chamber) and GFO for a relatively easy solution.
SG 1.025 -the best thing I can recommend is consider purchasing a refractometer; if this is not an option, I suggest getting your hydrometer calibrated using another reefers already calibrated refractometer (doesn't do any good to calibrate your hydrometer with an improperly calibrated refractometer).
My .02, HTH.
