Aquaponics is definitely a future project for us. I have gathered a lot of info on useful sensors etc for aquaponics/arduino but it was several years ago.
There is a lot more available now (for lower prices too )
The IA Site listed a Chronodot clock -- an alternative to that would be a GPS (they are under $15 on ebay) the GPS streams give local time and day and date -- this would be very useful for data logging as well as time -- there are programs to calculate sun up and sun down from GPS data -- useful for turning on and off grow lights for example
It is nice that there are chemistry sensors in a reasonable price range now too.
Since we are in WI (Driftless Area) the cold water species of fish would probably do better --- in Canada there are aquaponics operations that grow Arctic Char (MMMmmm)
I still think however that growing freshwater prawns would be a money maker if we could keep em warm long enough to get big (~6 months IIRC)
Nick, you can transmit data wirelessly using Xbee's
all for now, but this will be an interesting thread