Emperor Angelfish (Pomacanthus imperator)

"Minimum volume" indicates the size of the tank needed to house this species under optimal conditions.
This is based on a medium size animal, which you want to keep for several years.
It might be possible to keep smaller specimens for a limited period in a smaller tank.
A larger tank might be needed for fully-grown specimens.

"Hardiness" indicates how resistant this species is to disease and how well i tolerates bad conditions in general.
Some species doesn't handle transportation very well, but that doesn't mean that the species isn't hardy under the right conditions.

In this case, a "normal" aquarium is a reef aquarium with mixed corals or a fish only aquarium with an approximately salinity of 1.026 (sg) and a temperature close to 26°C.
Species requiring more than a 4000-liter tank are considered not suitable for home aquarium.
Special aquariums may cover tanks with low salinity, sub-tropical temperature, deep sand bed, sea grass etc.

Always reef safe: No sources indicate that this species will harm corals or other invertebrates.
Often reef safe: Only a few aquarists has reported problems keeping this species with corals and other invertebrates.
Reef safe with caution: This species may be a threat to some types of invertebrates.
Reef safe with luck: Most specimens will harm corals and/or other invertebrates, but you might be lucky.
Not reef safe: This species is a threat to most corals and/or other invertebrates.