There's a good chance it's an upside down catfish! I've seen a pinkish purple one before, they are born as normal swimmers and gradually flip upside down and pretty much stay there!
I can't remember the Latin name a sydontis (?) of some kind?
If it spends most of it's time on it's back and only inverts during feeding time, it's most likely an upside down catfish, synodontis negriventris, however, it it spends little amounts of time on it's back it could be another type of syodontis catfish.
Most Synodontis catfish have the ability to swim 'upside down' but some of the members of the genus prefer this meathod of swimming more than others.