the more bubbles/oxygen from your air pump and air stones in your water the better if the water temp is to high the water is unable to hold oxygen (see 'Water' and 'Hydro' chapter in the Indoor/Outdoor Marijuanna Bible by Jorge Cervantes Luck! :)
possibly hermies could develop sometimes with extra light time in flowering buds get more leafy and harvest weight is considerably lower sometimes you get lucky and everything is groovy good luck! :)
you can also treat powdery mildew with fungicide or naturally with sulfur. Sulfur comes in sprays, dry powder for dusting or you can pay a bunch for a sulfur burner. Personally I have found clod pressed neem and sulfur spray to be the best. :)
It sounds like your humidity is quite high for flowering... with an enclosed system like a hydrohut you might consider a dehumidifier set up outside your hut and piped through one of the ventilation openings this will save you a lot of problems with white powdery mildew now and bud-rot later on...
in soil give your plants enough water so that 10% of the water drains from the bottom of the pot this helps dissolve and leach extra nutrients and salt buildup from the soil
the first set of leaves (cotyledon) always turn yellow and wither as the seedling grows more nodes and sets of leaves
---wish you luck with your grow :)