The upside down fish

My aunt has 3 fish tanks... with various fish in them...but there is one fish which I find fascinating: and that is the 'upside-down fish'.

Normal fish

The 'upside-down fish'

At first I thought he was dead. He/she/it was lying motionless at the bottom of the tank (strange), belly-up (can't be good). But then it twitched....flapping it's little fins...as if to say hello. So, everytime I came back from somewhere I'd have a look in on the upside-down fish. He sometimes moves himself around the bottom of the tank and one day I actually saw him make a break for the top....which was weird to watch, as he swam, upside-down, in a flurry of activity to get to the top and then quickly sank back down again. From time to time the other fishes in the tank say hello to him... I came back one time, to most of the fish huddling around him.