with the associated surface water flow and aquascape
Pomacanthus imperator tends not dwell in the top 1/3 of the aquarium but rather down near the bottom.
This is why aquarium footprint is more important than total gallonage of the aquarium when giving a recommendation for the species.
Imps aren't the swimmers that many fishes (such as Naso spp. Tangs) are.
A 'bolt hole' (or cave) should be provided.
If fed properly this species will rapidly outgrow a small aquarium so (for many reasons) it's best to have a large enough aquarium prior to purchasing the fish.