about the live feed…

my db is about 3″ now. since i have had her, i have always had the pretty small feeder goldfish in the tank and she never bothered them. recently, if i put one in there she is very curious and chases it to eat it. it’s not like she is hungry or anything! i feed her daily. i think she may be bored. the other day i walked in and there was half of a goldfish body floating in the water! is there anything i can put in the tank to give her something to do? also, i was initially told to feed her as much as she will eat in 15 minutes, but she eats the food sticks so fast she doesn’t even take the time to eat one before she is after her next food stick! how much is good to feed her??