Which fish first

[Prariecanuck]

Don't worrie about me stealing your tangs yet!!! Still gotta build my tank first.lol
I was at the Lfs yesterday and now my list might have to change cause they had a small powder brown that was absolutely stunning and eating good I couldn't pass it up I had to get him. So now I have to get going on building my tank. What depth you guys think I should do would 20inch be deep enough for tangs?
Or should I go deeper? I like being able to reach the bottom.