I'm suprised you have no NH3 or NO2 and cloudy water. Could it be particulate matter? Did it happen suddenly or is it persistant after the water change?
High PO4 will inhibit a coral from depositing calcium in its skeleton. I don't think PO4 will lower your calcium level, just your coral's ability to use it.
1. Bring your Mg up to 1200-1300.
2. Bring your dKH up to 10.
3. Raise your calcium to 380ppm min.
I would do these steps over 3 days. It can be long and frustrating to get this balance perfect, and it will always need adjusting. How large is your aquarium and how heavily is it stocked with coral? It's much harder to achieve a balance with a smaller tank. You coral may also be consuming your calcium at a very high rate if it appears you can't get the level up.