I think you are referring to blue bottle fly's. I don't think they would be better than bumble bees, but they would complement the work the bumble bees are doing. Also good air circulation would help a great deal for pollination too. How are you dong/monitoring your
nutrient solution? Have you changed it yet? Are you replacing the
nutrients/elements the plants deplete from it? If so what are you adding and how much? Also what size is your reservoir.