Yeah I have to agree with OldMan47 there. 15 minutes is a very long time for the fish to eat, espically twice a day! If you do want to feed twice a day then little and often. As much as they can eat in 1 minute tops - 2 minutes if you feed once a day.
Try to avoid falling in the trap of feeding the tank until every fish has eaten as you will overfeed badly.
Remember, your feeding the filter, not the fish, so your looking to feed as much as the filter can handle while still providing enough nutrients for the fish.