Well I need measurments like is it 4' tall and 5' long or what? Because if it is not at least 2 1/2' tall and 3' long no angel can go in there. Because angels grow to 8" tall in fins and 5 or more inches in length. You would need at least a 29 gallon for one angel.
A dwarf or honey gourami would be a nice addition depending on the type of tetras and how many there are. You could have a single fish or if there's room a male and 2 famales of the same variety.