Depends on the size of your plumbing. I was using 1/2" ID
pipe, which I thought was pretty common practice, yet I couldn't find a 1/2" ID
check valve anywhere. You'll need to go to a plumbing specialty place or order from an online aquarium supplier.
You're describing a situation I call "reverse siphon". When the power stops, the pipe that normally sends water INTO the aquarium suddenly becomes a siphon and starts pulling water OUT of the aquarium. The check valve prevents this.
The other suggestion is to drill a hole in the pipe just as it enters the aquarium. It has to be a pretty large hole. The thought is that when the pipe starts to reverse siphon, it will only drain down to this hole. Once it hits that point, the hole sucks air into the line breaking the siphon.
In my experience, that hole has to be pretty large to interrupt the siphon action. In my case, the hole would suck in a lot of air, but the siphon would merely slow. But if the hole is large, a lot of water shoots out the hole when the system is running...
Finally, the ultimate security is to only have enough water in the sump so that the system can reverse siphon all it wants, the sump will still handle it. This is controlled by the water level in the sump and the depth of the return pipe in the aquarium. Run less water volume in total...and alter the depth of the return pipe until you've got it set.