Help Me

[Dobber]

okay so I am relatively new to the business of marine aquariums, well ive had my 90 gallon for two years and just recently upgraded to a 220 gallon corner overflow tank running two #3 megaflow sumps underneath. I have two 1800 gph magdrives for main system pumps and I have two koralia 2 for some circulation ($). I am in the process of trying to get a wavebox and some proper circulation. I have an aqualight pro 250w 72" with 10,000K bulbs. The temperature in my tank is fluctuating slightly between 74 and 79 not usually that big however. Okay so finally to my problem. I CAN'T KEEP CORALS AT ALL the only thing i have been able to keep is a small xenia frag that has only been in the tank for two days or soo lol. I feed live phyto once a week. I also feed frozen brine, cyclop eeze, marine plankton frozen cubes, frozen rotifers and marine algae flakes everynight. My fish however are gorgeous I have a Powder Blue Tang, Yellow Tang, Convict Tang, Juvenile Vlamingi Tang, and a Desjardini Sailfin, Regal Angelfish 7". Some other fish include a helmut gurnard prawn goby, purple firefish, 6 green chromis, six line wrasse 5", two true percula clowns, and I have a purple reef lobster that is well fed, I also have 2 purple queen anthias and am hoping to get more females. Also have a breeding pair of mandarinefish eahc about 4" long. I have a decent skimmer rated for 200 gallons, a 40 watt UV sterilizer and a reefugium that fills the second megaflow. My Parameters are as Follows.

Salinity: 1.023
PH: 8.4
Calcium: 400
Carbonate Hardness: 90
Phosphates: Barely Detectable
Nitrates: Very Little
Nitrites: 0
I have also ben adding strontium because when i put corals in its almost as if the flesh lifts right off within the next few days.

PLEASE HELP