I take it you have a level indicator on the side of a tank outside, showing some variety of tank level conditions, and you'd like to attach a sensor to an Arduino to read them.
If the Arduino is at some distance from the tank, finding a suitable optical sensor that will read weak LED signals at a distance, in all sorts of lighting conditions, probably won't happen. If you are going to mount the Arduino on the tank, then you might be able to get some photoresistors, or phototransistors, wired up as half a voltage divider, and read the voltage divider output into an analog Arduino pin to determine lit vs. unlit.
Of course if the LED's are multicolor units, or signalling by some sort of blinking pattern, the problem gets tougher. If you can get to the leads of the LED's you might be able to detect at that point, but be careful about voltages and possibly isolation if there's AC on the indicator board.
You should provide more detail about equipment placement and signaling protocol.