Tri, while I understand your passion for livestock, please consider what you are doing. I had 6 tangs in my 280g reef, and felt like they were too much for the tank. I'm down to three now, and still considering removing one more. Tangs swim for miles every day in the wild, foraging for food. That is one part of Finding Nemo that is accurate, the way Dori swam off into the mist, virtually out of sight.
8 tangs in an 80g is too much. Since you plan to upgrade to a 180g, it is time to stop buying anything else, so you can fund that larger tank. At least this will give your current fish more swimming room, which I know you want to provide. It would be best if you didn't add any more tangs even to the new tank, because they do produce plenty of waste. The six tangs in my tank definitely added to the phosphate issues I was battling for six months.
Btw, it was you that brought up this topic, so don't feel attacked. We care about the livestock's welfare, and speak up if we are concerned. :?