Thanks for your involvment! You've made me think twice about some things which is great!
True enough!
You're right about gravity
A malfunction will almost every time mean that the timer or pump stops working. The solenoids will open and fill for some minutes and it will just overflow. But if the pump/timer, against all odds, would run crazy and not stop pumping then the tank would drain completely and the fish die. Sad of course, but still no water in my house.
Ok, I don't think we agree on this one. The water volume in the tank is a risk coupled with every aquarium. The big difference with an automatic system is that you have the possibility of an unlimited water volume flowing into the house (as with all other water pipes in the house, except the insurance company don't like DIYs). Since this unlimited volume is solely in the fill line, then I think that's where the reduntant safety measures should be. In my set up that's two normally closed solenoids very close to the water source, plus the float valve. Even if a fitting in the fill line blows off it will most likely be behind closed solenoids except during filling.