Lots of options here. Chocolate is a good choice, many of the viejas will work, red devil, Texas cichlid, loiselli, Uaru, or carpintus.
If you really want a wet pet it is tough to beat a puffer, but that is definitely out of scope of what you asked. Another one that is extremely interactive, but out of your scope is an archer (clouded is best). It would be a little weird to have just one in a 75, but it would work.
Out of all of those fish my choice might be the chocolate. They are really interesting and they seem to change colors with their mood.