You should only give what they can finish in less than 3 minutes. This is more important than how many times a day. The biological filtration is much more effective at breaking down fish waste than it is with rotting food. As any fisherman will tell you, dusk and dawn is when they feed in nature. There are different considerations with a closed system. You want the fish waste to enter the filtration when the lights are on, so IMO, don't feed them past 3 hours before lights out.