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[maitaman]

The contamination will generally show before five days, so that's the delay.
The orchid moves from a stage where it "feeds" on the sugars in your solution, then moves to a chlorophyll/carbon dioxide cycle after the protocorms start to form roots and leaves. This is why a mother flask doesn't need venting, but a growing flask must have it. The plantlets grow much faster in the solution and are protected from the normal pests and such that nature provides. It is faster and better all around to leave the plantlets in the flasks until they are getting fairly large, then they do best in community pots for awhile, then individual potting.
Also remember: if the plants are in the flasks you don't have to worry about water, insecticides, fertilizers, and numerous other critical things that have to be considered when they are not in the closed system of the flask.