I took the white plastic off the sensor and measured the light outside the tank.
I noticed measuring in the tank full of water was giving me higher readings that didn't fall on the inverse law curve.
over all cost:
~$10 - photo diode + S&H
~$.50 - plastic paint cup
~$4 - electrical cord
~$30 - ardiuno MP (there's a $20 version too, and there's a $5 texas instrument one)
~$.50 - 27K resistor