oh boy..could be a number of things. But mostly, I'd say your nutreint levels are too high. You may change your phos out sooner. If you run you phos to long it can create higher levels of nutreints. Try changing it once every 10 to 12 day's and change your return filter every third or fourth day. All this of course will depend on how much you feed your tank. You may want to step up your water changes to put back trace elements that the corals are useing up. Use a good quality salt, don't use the cheap stuff, your results will be "you get what you pay for". I would recommend doing weekly water changes of 20% for 2 months and then step back to every two weeks for a couple months and then once a month. You will notice a big difference in your corals and you'll start to have success. Like anything, start off with a good foundaton and keep good husbandary. It's worked for other people haveing the same problem your haveing. Oh yea, I would agree with the statement above, watch for big temp swing's and PH swing's. Anyway...that's my two cents.
Good Luck