In Groton, we're learning about diffusion and osmosis in class.  We used the opportunity to talk about dissolved oxygen in our tank and related the concept to our Brown Trout gills.  If you've never used Brainpop, they have a great video on fish gills and countercurrent exchange:  http://www.brainpop.com/science/diversityoflife/gills/  Give the free trial a shot.  On another note, all of the trout seem healthy and active.  We've only lost about 8 fish since the beginning.