Actually I am using a sanguino for the same purpose... I use 500 gph bilge pumps to control the ebb and flow tables , and an inexglobal.com Relay16 board attached by i2c to control pumping and control the lights via Solid State Relays mounted onto heat sinks.. also http://www.nano-reef.com/forums/index.php?act=attach&type=post&id=91001
is an artificial reef controller arduino bootloader compatible design that incorporates isabella's PPM and PH measurement circuits with a 644P processor. Turns out artificial reef control and control of Hydroponics nutrients are almost the same measurement problem.. also try search keys of growduino on google..
good luck with your project
ps dont forget the k88 CO2 sensor i2c interface thread in these forums..
think DS1820/18B20 temp sensors in a network(one wire)