Clowns shouldn't be kept in a 5g, period.
Most that suggest keeping them in smaller tanks, either have smaller clowns, or have never seen a full grown one in person.
I have a pair of false-perculas, one reaching 5 years, the other reaching 8-9.
Both live in a 55g, and are pretty active.
If you placed either in a 5 gallon, they would sit in the corner, and wouldn't move.
I would suggest a 30 gallon, or at least a 20g long as a minimum.