Algae needs light to grow. Ensuring no light makes its way to the nutrients through the bucket is the best way to prevent algae growth. Algae also thrives in anaerobic conditions (lack of oxygen) so be sure that plenty of air is being pumped into the buckets. I dont know the minimum amount, but you should not have any problems if you are pumping in at least 500 cc's/min of air per gallon. 600 cc's/min per gallon will ensure plenty of oxygen is being introduced in the nutrients.
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