very healthy fish! Also glad to hear you are taking it back, even though its hard and youve obviously cared for it well.
the reason you don't add gallonage from a sump into your stock plans is because the fish arent utilizing that space for swimming. most minimum requirements are set by how active the fish is. tangs are fast and swim long straight distance, therefore need long tanks. putting one in a 10 gal tank with a 400 gallon sump does not give her the space she needs to feel comfortable (obviously exaggerated for[ show). if you are talking about something that needs very stable waters such as some echinoderms you can include sumps because that offers greater stability and they dont need a lot of space, just a lot of algae...