Once filled with water, those ACs are supposed to self-prime. That is, start up on their own, as long as the water level is above the tip of the intake tube. Even so, I find I nearly always have to help mine along after a water change by scooping tank water into the media box and the impeller chamber. Before I do it, it just sits there and goes "grrrrrr." After scooping water into it, it begins the familiar "chug-chug-chug" and primes up and goes silent once the waterfall starts.
That's why I've always feared they might not start up again after a power outage, when the juice comes back on. Apparently, if the water level hasn't lowered, it's not a problem, but after a water change, even when the tip of the intake tube remains submerged in water, it's not high enough for the AC to prime itself. Once in a rare while, it will, but usually it needs my assistance.
20 gal. high: planted; 1 zebra danio, 6 glofish, several snails, 2 (visible) RCS; AC50, Azoo air. 65 gal: planted; 4 rosy barbs, 6 glofish, 5 white cloud minnows, 3 zebra danios, 5 dojo loaches, several snails; AC110 x 2.