don't worry sis... its normal.. evene if you don't feed them so much they will still keep finding food to eat...
if i'm not wrong they eat so much because they are herbivourous(forget the spelling ) and living in a salt water environment they need alot of energy to expel the salt from the water... thus explain y they keep eating and eating and eating....
and IMO other than eating, searching for food and mating... i seriously don't think that they can do other things... haha....