You may be surprised at how diving is done here - I was when I first came here from California. I was used to going in the water with my dive buddy and going off in what ever direcion we choose, then returning to the beach of boat.
I have not found any dive operator here who does that. Someone correct me if I'm wrong. We have a five to ten minute ride to the other side of the reef, enter the water and go down as a group and follow the dive master as he leads us. Most dives are for 45 minutes which includes a three minute safety stop.
We do 45 minutes to an hour surface interval and repeat the process.
On the first dive we may descend to 80' or so then work our way back up. Most of the life is between 40 and 60 feet. Safe diving calls for us to, on our second dive, not descent lower than our deepest depth of the first dive.
That being said, I have occasionally been allowed to "stay behind the group" if I have something particularly interesting to photograph.
I'm sure we are all curious about why you want a large tank. Unless you are a gulper or hot dogger a 80 psi tank lasts the dive with plenty to spare.
Take only pictures leave only bubbles