Moving food items should make a positive difference at increasing the number of larvae eating food particles. The ones you see "nipping" or displaying obvious feeding behavior are no doubt feeding. As you introduce food it may help to swirl the water as you dispense the food to make it move and swirl around as if it is swimming. It will not take very long to figure out which ones are eating because they will be getting bigger. You may also notice the largest ones eating the smallest ones.
If you do this again I recommend that you eliminate the gravel from the tank bottom. It contributes to increased poor water quality by collecting food items and manure. Accumulations of all types of waste materials in the intestitial areas make daily bottom siphoning less efficient. The more waste you can keep out of the tank the better the water quality stays. Wastes as they decompose produce ammonia - a killer of fish.
