You really need to get that nitrate down. Look in the Marine forum here about my vitamin C dosing. Is there a protein skimmer, refugium? Definitely more, larger WC to remedy this. Try offering him live earthworms.
pH should be stabilized by substrate & salt. I would not use a chemical for that. I'd also highly suggest purchasing a calibrated refractometer. I happen to have an extra one I'm selling that comes with calibration solution. You really need a much bigger tank for him, too. They grow fast!