Looking for a way to detect dissolved oxygen levels? If you raise fish, this and a ph sensor are two important things to monitor (and of course, temperature), and an Arduino is the ideal platform to build upon. The Sensorex DO1200 ($139) outputs a <1mv - 54 mv signal indicating DO levels. Use
analogReference(INTERNAL1V1) to set the top of the input range to 1.1v.
Connect it to one of your analog pins. Very simple to read, just like a potentiometer.