Noooo!! no more powder blue tangs! They are one of the most difficult fish to keep along with the Moorish idol and also wrasses aren't the easiest either. Maybe stick with more common fish like clowns, chromis, royal grammas, etc... Its really not normal for so many fish to die under ones care like this. Work out the bugs with hardy fish for a while, then if you don't have any deaths for a long time, move on to something maybe a little more difficult.
You also need to get on a better water change schedule. Say every 10 days or so. And stick with that schedule without fail. And if you do a big one each month...stick with that without fail. You'll be amazed at what a schedule water change plan will do. Reef tanks adjust to schedule. It knows when you miss one here and there and it doesn't like it.
Also, a 30 day quarantine should be minimum every time.