Ill have to do some tinkering this weekend. thanks for the ideas. ill post back with some results
Okay, here's one. Have a floating disc on the surface. On the container rim, mount one of those little lasers pointing at some angle. Assuming the floating reflecter stays flat, the reflected point wil move up a side wall (draw a picture, you'll see), where you have a series of detectors (string of light detectors? like LEDs being used as sensors?) that are calibrated to the tank volume.
Will it work if you skip the floating disc, mount the laser under the surface and use the reflection of the light with flat angle?
One more idea. -> will sure work. Get a Pressure Sensor e.g. BMP085 (there are Break out Boards)
Put the sensor inside of a glas Bottle.
The sensor will be mounted on the bottom of the Bottle, cable can go through the opening.
Then push the bottle up-side-down into the water.
The Waterlevel that is higher than the opening of the bottle/tube is related to the pressure.
It can be calibrated or calculated. -> if the bottom of the bottle is higher than the border of the tank the sensor stays dry.
Underwater laser - maybe not in this case:
"Its a cooler with wheels. holds about 5-6 gallons of water. Its holding hydroponic solution."
Sounds plausible for cases with clear liquids.
Pressure sensor sounds like good idea too.
hmm I like the pressure sensor idea.
As I use commercial hydro solutions they are fairly clear. Slightly reddish color but its not muddy looking. So the laser could work. Sounds cool too. Fricken laser beams.
I think those were thought of up above somewhere. The issue would be waterproofing them and them still working. Im a big fan of globs of silicon 
Looking at that link though gives me a few ideas. Course now I need to learn to cut and glue plexiglass...
I've researched this quite a bit. Range sensors aren't accurate enough; there are commercial ultrasound units that sense water surface, but they're very expensive. A sensitive pressure sensor can be used, just run a tube up from the bottom of the tank to the a point well above the highest water surface and use a differential sensor. Rising water pressurizes the tube; you don't have to worry about waterproofing anything (which is always a pain and never lasts :-). There are units now on Mouser et al. with ranges of 12" and less.
cool, thanks for the tip.
oh.... i am worry about next month..
soon i am going to get the bill for water.
if you just need several levels (low med high) then i think i would just use float switches like these mounted at the heights where you want to read
http://cgi.ebay.com/10-X-Liquid-Water-Level-Sensor-Right-Angle-Float-Switch-/190468077770?pt=LH_DefaultDomain_0&hash=item2c58c838ca
i have used the stainless versions for some waterfall projects and they seem to work well
if you want to know the exact height then you may try one of these or go to a junk yard and see what they have that would fit your tank
http://cgi.ebay.com/ebaymotors/TUBE-STYLE-fuel-tank-SENDING-UNIT-universal-NEW-/300493749205?pt=Vintage_Car_Truck_Parts_Accessories&hash=item45f6d2bbd5
i also like CrossRoads idea with the strings with floats that would pull micro switches as the water level falls
you know i looked at those a few times but by the picture I cant tell how they work. is something else attached to it? You got a pic of one in action?
http://cgi.ebay.com/10-X-Liquid-Water-Level-Sensor-Right-Angle-Float-Switch-/190468077770?pt=LH_DefaultDomain_0&hash=item2c58c838ca
I'm guessing that the donut part floats up and down the short shaft, it has a magnet inside that activates a read switch in the long shaft.
Rob
they are quite simple there are only 2 wires
there is a magnet in the float and a reed switch in the shaft so when the float floats it opens the circuit and when the level drops it closes it
the last ones i got were stainless and you could flip the float upside down if you wanted it to be reversed (closed when up and open when down)