The best way I've found for mixing tangs is keep the tank completely dark for the first 1-2 days except when feeding. Then slowly start working your back up to your normal light schedule.
Why is everyone so persistent that fish need a 2-3 hour acclamation time? I've never done it that way and never lost a fish due to my acclamation process. I think the 2-3 hours you keep them in that bag just adds more unnecessary stress.
I agree with Beth that it's most likely aggression...
I don't think those will be strong enough you should look into orbit marine leds or some of the cheap chinese boxes.
Also be careful about posting links to other stores.
I don't think its the acclamation process that's killing the fish especially if it's clownfish. I always just float the bag wait 15-20 minutes then let him go I've never lost a fish doing this.
Do you make sure the fish are eating before you buy them?
I don't mind it if he wants to eat the algae I just like that he's sharing food with no aggression as all. I tried to pet him this morning he's not ready to let me yet
Pretty sure I spent an hour watching him. He let other fish come into his cave, he ignored everyone when they feed him. They said they've had for over a month with no issues. they also said if he does get aggressive they'll take him back.