I can't comment on the plumbing.. But I understand the badly drawn electronics schematics.
Actually, it's not that bad. You're powering the relay coil through a resistor and an LED or two. When the sensor is tripped (closed), it provides a lower-resistance path (and lights a different color LED), which lowers current through the coil enough to deactivate the relay.
Now my question is this... How are you going to implement that sensor? Aquarium water isn't all that conductive.