Here is my idea for an auto top-off system. I haven't actually made this system yet so I'm not sure it would even work at all.
Basically it just consists of a water reservoir (preferably an air tight container to reduce evaporation), a water pump, and two liquid level float switches wired into the pump's power cord.
Note: The tank's float switch must be mounted upside down or else it would always be in the "ON" position and you would have a big mess to mop up. The water reservoir switch should be mounted as pictured in the drawing and always be in the "ON" position. This switch is optional and is only there to prevent your water pump from running dry once all the water in the reservoir is gone. When the reservoir is empty, the switch would cut off the current and the pump won't turn on until you refill it. You should also make some sort of casing around the float switch in your tank to prevent it from going off by mistake. Snails and other inhabitants have been known to set them off and flooding the room. You might also want to add a check valve or something to prevent water from siphoning into the water reservoir. I'm not sure if they make some sort of check valve for liquid but a regular airline check valve might work if you use the right size tubbing from your pump to the tank.
If you see any problems with my diagram and/or have any opinions or suggestions feel free to post your thoughts.
I personally don't want to try this myself because this is a 110v setup and I'm afraid of frying my fish. If anyone here knows how to make this a low voltage system please let me know.
Ps. I am not responsible if you attempt this DIY project and hurt yourself or your fish.