Mains socket transformer that the air pump and light plug into.
biOrb transformers are very reliable and rarely need replacing, if yours has stopped working and you have a standard light unit check that the last bulb you fitted was the right size. If you have fitted a larger bulb it will 'blow' the transformer and stop it working. If this is the case replace the bulb before plugging into your new transformer.
Suitable for all sizes of biOrb & biUbe aquariums.