help

im trying to set up a salt water tank its been two months now and i have had trouble before but this time is different. i have a 45 gallon tank and i do 15 gallon water change every week im still high on nitrate and a little on ammonia i have had fish in the tank for a while now but all the sudden things started to die i had a blue tang yellow tank and two fire fish and a spotted goby alone with a marron clown my yello tank went first and then my blue tang AND THEN one of my fire fish the levels have not changed if anything the have gone down the feeding is normal i feed mysis shrimp and then sometimes i feed the maroon clown flakes because he is always so hungry nothing around my tank get sprayed and i haven't gotten anything new lately and these fish are just to expensive for me to buy again if they are going to die i don't have to much experience but i do know a lot any help please i need help