Hi ckyz,

If all you need to do is detect the high and low levels in a tank, then something like these floats would do the job.
http://cgi.ebay.com/ws/eBayISAPI.dll...m=170007230942
$5 each.

However, if you want to display the actual level in the tank, you'll need a "Continuous" level sensor. Here are a couple examples, but these things are usually several hundred dollars each.
http://www.pulseinstruments.net/inde...&Category=1094

The idea is to bounce an ultrasonic signal off the water and measure the return time. Much the same as the little sensors everyone puts on their robots to detect obstacles. Here's one that might work, as long as your tank is less than 3 meters tall. Should be good for starters.
PING))) Ultrasonic Sensor
http://www.parallax.com/detail.asp?product_id=28015
$24.95
<br>