Would like to add another fish but not sure which is best.

[riotgreen]

I would like to add another fish to my tank but not sure who would fit in. I have been told I can add a tang or angel. Any suggestions welcome. I have a 75g with a clarks clown; humu humu trigger; niger trigger; blue damsel. Filtration is two fluval pumps and has been up over a year. I also have three horse shoe crabs and plan to have a ribbon eel.