Back to the Darter, we can debate the merits of capturing natives in another thread. I agree that the darter needs very clean and cool water. If you have a heater yank it after slowly turning it down. Don't know thank size, I used a 20L, but you need to hang an HOB
filter on one end so the water runs the length of the tank. You also must have some flat stones large enough for the darter to hide under or it will eventual surcome to stress. As for food, live baby brine shrimp and micro worms are fine. You can forget what you have tried and don't waste your time and tank stability trying to convert them. You will need a few other fishto clean up uneaten live food.
If you really want to do your project correctly and have some very cool fish, look up Jonah's aquarium. They sell what you need.