Wow, thank you all for the very quick replies!
I guess i really wasn't clear enough about the project itself:
-This will control the water flow in a homemade ebb & flow irrigation system: basically the water is being pushed up through the BOTTOM of the bottles at set intervals per day.
-Once the set level in the bottles have been reached , the pump shuts of and releases the water BACK into the container.
more on it here :http://www.hhydro.com/cgi-bin/hhydro/ebbFAQ.html
-some picture for more clarity (on these photos i only linked up two bottles but normally it's four)
bigger versions just in case :
Grumpy_Mike:
-thanks for that link, i have read it but i will do so a couple more time to understand all the concepts:)
-well i am running it at a lower voltage : normally it is a 6-12v pump, but i tried it at 12v and not only is it way too fast , but it also almost fried the tip120
-to get the Amperage i measure the resistance of the motor (when not running it is about 2.2 ohms), and then i divide the actual voltage used by the resistance
-yes sorry about that it is a lower valued resistor :100 ohms
-i will take a look at the Darlington pair thanks for that 
-i am not really sure what way i would have to measure the water flow, as the tubings are 1/4" ...hmm although, now that i think of it i could perhaps calculate the flow based on the elapsed time between two set level of water..i will study this !
RIDDICK & FusiveResonance
-you are both quite right, that is indeed a possible solution
-given the requirement in the system i think i would need to add one such electrode per bottle and use any kind of "logical OR" operation to gather the input of all the bottle and switch the pump off if the water in ANY of the containers reaches a critical level
-i don't really know if there would be a danger of running 5v though the water but i doubt it , if i remember right i have seen EC meters with way higher voltage
-actually i even think i might be able to use this electrode system to both measure the water level AND monitor the electrical conductivity of the water ...then again, if the resistance of the water changes...hmm
-the float switch would unfortunately be very impractical in this case (see pics)
-wow, didn't know about the PID controller, but i'll definitely have a use for it, since i will need to monitor and control a lot of other parameters thanks:)