Mosquito larvae are filter feeders and feed on very small food particles(bacteria and algae mainly). So even powdered fishfood is probably too large. They prefer water that is borderline septic providing it doesn't have a thick surface film(from what I've seen in my 5 gallon bucket mosquito baby abductors...the smellier the better!). Of course, you don't want that in captivity. The fishfood may decompose and give the larvae the food they need. I haven't tried growing them intentionally but I think a drop or two of the liquid leftover from canned tuna, liquifry, a few drops of soup, green or cloudy water(such as you might make for a daphnia culture), or similar should work. I have noticed what you saw when storing them in small containers, it is likely an adaptation to allow at least some of the larvae to survive in the wild where they may be faced with the water drying up. The easiest thing to do may be to grow the larvae for a few days, then add them to the larvae tank even if they are still pretty small. IME they find enough food to grow and are cleaning the water in the process. Keep the thing covered and don't contribute to your neighborhoods mosquito population!