If you're thinking about a porcupine puffer, I would advise against that. Unless you want a big swimming stomach in your tank. I unfortunately inherited one with a tank I got from a person who was giving up the hobby. It would greedily and immediately consume any amount of food I put in the tank and was constantly looking for more. It would even eat caulerpa algae from my refugium, so I'm pretty sure it would have eaten Tang veggies as well. I quickly got tired of it eating all the food and generally making a mess of the tank, so I took it to the LFS and gave it away for nothing.
I see you inquired about the Moon Wrasse in one of your posts. I also had one of those. It was really beautiful when it grew to its full adult size, but was a very agressive feeder as well, at times going so far as to take food right out of other fish's mouths. It also had the somewhat annoying habit of spending all its waking hours swimming back and forth all over the tank with a herky-jerky motion. This lack of respect for individual territory seemed to stress out some of the more mild-mannered tankmates. This guy got too big and mean for my tank and wound up being another freebie for the LFS. I have a lime green wrasse now that's almost as pretty, but much less aggressive.
I would like to give something other than negative advice but I've never had a tank that big. I've always dreamed about having one big enough for a miniatus grouper, because IMO it's one of the finest looking fish there is. I think a 180 would qualify, so if I was you, that's the fish I'd be looking into. Of course I have no idea whether it would be a good choice or not, having never kept one. I just think groupers are really cool.