A particular point about these fish (and others in their family) is that they don't have a swim bladder - if they don't swim they sink!
This is why they can and will lay on their side/back whatever. Most 'normal' fish have a swim bladder which means they can control their bouyancy in the water - float mid-water, remain upright etc.
Oh and another thing, they don't have scales either, which means if you use any medications you should pay attention to the amount used - in some cases it is appropriate to halve the normal dose (can't remember which, by the way!)
Top fish - my favourites by a mile. If you have room in your tank you should get a couple more!