Do you need to know the exact level? Or just if it is below a certain level and needs some water added?
This is an inexpensive and non-intrusive method for detecting fluid level at a certain spot. I used a tube for this application, but a vertical corner of an aquarium would deflect nicely too when filled with a fairly clear liquid.
Here is the setup: A 5v power supply, a LED laser, a 5/16 tube with a sample in it (gasoline in this case), a cds cell, and an ohmmeter. The tube is flat against the board and the laser had to be propped up a tiny bit in front to hit the CDS cell through the empty tube.
The tube is empty in front of the laser and the CDS cell is well illuminated.
It is about 1.5K ohms here:
The board is tilted and the laser gets deflected by the fluid in the tube and misses the CDS cell.
It is about 11K ohms here:
So there ya have it. One just needs to carefully drill a couple holes in a block of wood to organize this assembly. But with a jump from 1.5k ohms to 11k ohms it should be pretty easy to detect.
Here is a cheesey picture of what is going on. Note that you have to adjust for the empty tube when aligning the laser, it does not just go straight through! Also, the farther away the CDS cell is from the tube, the greater the deflection distance.