500 is certainly high enough , i also dose with brightwells ,strontium ,magnesium ,iodine and koral color which has really made a big difference with my corals . i also dose calcium and alk as most do anyway . along with 20% weekly water changes to keep everybody happy with reef crystals made with ro/di water .
Tinymouse ~ i really dont know as all of mine seem to be very extended but every now and then a few heads on one of my colonies retracts for a couple of days and then snaps right back to normal . you could try to dip the effected coral and see if that helps out . every new coral that i get gets an iodine based dip for ten minutes and then an interceptor dip for 24 hrs to ensure no hitchhikers are making it into my tanks .