I have 3 fish in my 14 gallon nano. Royal gramma, clown, and rolland's damsel. The gramma hangs out in a cave at the back of the tank and only comes out in the evening to feed. The other two stay in their respective territories too, so in general, the tank looks pretty empty aside from all the coral.
I have no algae issues, and my corals are happy, so I have no nutrient issues. I figured I could probably add one, maaaaybe two more if I really wanted, providing they were nano fish. However, I don't see the point in adding fish to simply add fish; it has to fit in nicely with the overall scape, and I just can't think of anything else that would work. Gobies hop on corals and cause them to close up, and wrasses (such as a sixline) would just eat my pods and microfauna. A toby puffer or maybe a dwarf angel (really pushing it here) are appealing, but both have a likelihood of eating my corals, especially since my tank is LPS dominant.
I will add though, that a lot of people put too much credence into the tank length or amount of potential gallons it could hold in regards to housing a fish than the actual environment it lives in; if your 5ft long tank is filled with massive SPS that take up 70% of the volume of the tank, then your tang really doesn't have as much swimming room as you think. /shrug