I'm no expert, YEEH, but I do feed my fish Optimal. It is one of the most expensive feeds, so obviously I feel the nutritional value is better than most. However, Purina makes good feeds as well. Either should work well for you.
Based on my experience, I'd say to begin feeding when water (not air!) temps hit 60 or a bit higher. Start in the afternoon when things warm up, as fish are more active then. Later in the heat of summer switch to morning and late evening.
It may take a while for the brim to figure out what's going on, so don't get discouraged if nothing much happens for a couple of weeks. They'll get it eventually! Just don't feed much until they do.
How long? Let the fish tell you. If they finish the feed in less than 5 minutes, feed more. If feed is left over after 10 or 15 minutes, feed less.
One benefit of feeding is the sheer fun of watching them eat. What a blast!
