Mike wish I could help you but never kept minnows alive more than a day or two..... where I fish now a days, only dead minnows are allowed.... I always keep them overnight in the fridge in the basement and when comes time to leave in the morning, drain the water from the plastic bag.
I think the trick is to keep them in a cold environment as much as possible.... the temps in the fridge certainly helps cause every time I go down to get a wobbly pop they go crazy LOL
As for food.... may be the worm based tropical flakes they sell at the fish store can help.... give it a try and see if some will come to the surface to feed. Now a days, all the ones I have left over get a good bath of pickling salt and into the freezer LOL
Good luck with keeping them alive for an extended period of time!!!
You got a nice aquarium setup BTW
Jacques