Water that has been enriched with nutrients and passed through a hydroponic system can definitely be reused. Care should be taken to not over-fertilize reused water because an unecessarily high concentration of nutrients will encourage the growth of algae and could damage sensitive roots. It's the ability to re-use water in this fashion which can make hydroponics a sustainable method for food production.
Since most hydroponic water is reused, aeration in reservoir tanks is an important feature to ensure consistent success with your crops. Aeration provides good dissolved oxygen content, which in turn supports the growth of beneficial bacteria, and reduces the growth of pathogens produced by anaerobic bacteria.
