"Play dead," fishy
The owner of this parrot cichilid suggests that he can order his fish to play dead on command, but we suspect it's a bit of an act. Parrot cichlids are one of a few tropical fish aquarist keep that will often exhibit this odd self-initiated tonic immobility. No one really knows why.
Another common type of fish that "plays dead" are loaches, such as clown loaches. Many fishkeepers panic when they see their loaches lying sideways only to realize after time that they're just being lovable goofballs.