I have always punched small holes in the plastic lid of my culture containers to allow for air circulation. This has produced a second food for my fishes as during the warmer months of the year I often find other small worms in the culture as well. This is because the odour of the culture will attract the common housefly, which lays its eggs through the small holes of the container lid. The eggs then hatch and the worms develop and grow on the culture medium. This may seem a little unpleasant to some people, but these worms are ideal for larger species of fish, which love them. Their development doesn't appear to have any detrimental effects on microworm production.