Sudden Tang Death Syndrome STDS?

Let me add to answer Mark's question....

Water surface interaction may not be very important at all.... in the natural systems.  But in our tanks, we have a heavy overload of biomass, and very little water motion.  So for our aquariums I think surface water movement is crucial to tank health.  This is one area where we don't want to replicate the ocean, we want to do better.

Adam

Fish gills are so efficient it is unreal.  I could explain the whole process of Countercurrent Gas Exchange, but just by the name of it I bet everyone has already tuned out this post.  Needless to say that fish gills are far superior to human lungs.  We breath in 21% O2 and breath out 16% O2.  They breath in water and breath out water (more or less).  To answer Mark's other question, there is certainly more O2 in the upper water column, but there is also more organisms using that O2.  There is less O2 in deeper water, but I don't believe it hinders the respiration of any fish.  There are greater problems to face at depth besides O2 concentrations. 

Adam