i have a tank thats been running for abt 6 months, there are 7 fish in there now that have been in there since the first month. now whenever i add fish they last abt a week and mysterously die. all my levels are perfect. anybody know? im pretty new at salt but have been doinf fresh for a looooong time. please help!

also heres some info on the tank
its a 120 gallon tank
2 250watt mh 15000k, 8 24in pc attinic
large wet/dry filter with 950 gph flow
large protein skimmer
fluidized bed filter
130 lbs of live fiji rock and 4 inch sand bed

ph, copper, calcium, nitrite nitrate, amonia,phosphate

Are the fish you're adding smaller than the fish in the tank now? Maybe there is a mantis shrimp or some other type of carnivore eating them.

no mantis shrimp. all the fish i have are 2inches tops. and the fish that died were all bigger. they all came from the same lfs. i have no idea if theyre wild caught or captive bred. i have oceanic sea salt mix. and the sg is right where it belongs. the odd thing is that the fish that i got 4 months ago are all doing fine and its just the new fish, they show no signs of distress, disease or attack when they die

Are they eating? I know the few fish i've lost never ate once i added them to the tank. Water levels are great, no killer fish or shrimp. They just wouldn't eat and perished within a week or so of being added.