Even with aquarium controllers its possible to fail. Controllers must have a set of contacts whether in a form of relay or sort, contacts can remain in the closed position therefore providing constant power to the source. I think that having two timers in series is more fail safe. If you have first timer programed to go off first and it does not then the second will. Generally contacts remain closed due to age of the contacts, whether they are pitted or carboned up.
Kevin
Kevin