Did the salesman tell you that the corals had to be fed? hmmmm....
some respond to feeding well, like the acans mentioned. Xenia won't take meaty food the same way, but seems to respond to phyto. I had trouble with xenia starting out 1st year. Now, I prune monthly. Keep temps below 84, keep alk steady, keep Ph steady. Add Iodine daily. Weekly water changes are great, but get expensive; once you find what your tank requires, you may be able to back off water changes, and simply add some 'vitimins'
Each system is like a kid- with its own personality. I have a 40g DT that gets a water change when I see build up of detritus on the substrate. Other than that I add Strontium, Magnesium, Kalk, Iodine, and top off with fresh water. I might do 2% a month- the water changes amount to how much vacuuming is required. It works for that tank, but not all tanks...the biggest lesson is that nothing good happens quick.