Feeding a red ear slider turtle?

how much do i feed him? the person that sold us this turtle in the store told us to feed him 10 pellots a day... but i read online that for adult turtles, only feed them every other day to every 2 days. i dont wanna over feed him... and im not sure if i am or not, cuz i think hes throwin up. i heard an air bubble, and i looked over and some stuff went floating.. little brown that looked like they had fuzz around it.. what was that? and how much is the correct amount to feed him.. i dont wanna make him sick..