Mains socket transformer that the air pump and light plug into.
Safety is vital when operating electrical items in close proximity to water. All aquariums need electrical power to operate their lighting and filtration systems and most use mains electricity at 240 or 120 volts. This can cause serious electric shock should the fish-keeper drop or drench the electrical items in the aquarium water.
The Reef One range of aquariums is designed to be extremely safe and therefore all its electrical items at the safe low level of 12 volts ac. To protect the fish-keeper and children from electric shock Reef One aquariums use a plug top AC transformer that plugs into your electrical mains socket and steps down the mains voltage to a much safer 12 volts ac.