My word of advice would be to plan for the worst case. Think through what would happen if any single component were to fail. Any time you're hooking a potentially limitless water supply to your fish tank you're creating an opportunity for a lot of water to end up in the wrong place.
For instance, if you use a system where a water supply line is running to a mechanical float valve, what happens if a snail crawls into the valve when it's open and jams it?
For this reason, I like to design systems that have some degree of redundancy or fail safe. In other words, if it breaks, it won't cause a flood.
There are a few common ways to do this:
1) Provide redundancy for failure-prone components. In systems with electronic float switches, use two, in series, such that if the "normal" one jams, the water will rise and trigger the backup switch, turning off the water supply.
2) Back up one control method with another. If your system is operating on a mechanical float valve, consider adding an electronic float switch above it, controlling a solenoid on your water supply line such that if the mechanical valve sticks, the float switch will trigger and shut off the water.
3) Limit the total system runtime. If you're using a pump or solenoid run by float switches to supply the water, plug the whole mess into a timer and set it such that it can only run just enough time to do the actual topoff, plus a short interval for variation. For instance, if it takes the topoff system 10 minutes to top the tank off each day, you might plug it in to a timer set to be on only from 12:00 to 12:15 each day.
4) Limit the total volume of water that can be added. Instead of tapping in to your home's water lines, consider adding the topoff water from a reservoir or storage tank. Pick a vessel large enough to provide a few days of top off, then fill it manually on that interval. This way, if the system fails, the worst that can happen is only a small volume of water getting spilled.