My first impression would be to use a check valve because the siphon would not be strong enough to open the valve, but the pump's pressure would be. This should stop this problem.
The kind of check valve that I am referring to is small and has a doo-hickie inside that looks like this:
The siphon should not be strong enough to force open this rubber doo-hickie.
Another solution would be to skip the aqualifter all together. You could just use an elivated reservoir plumbed to a float valve in the sump. The sump will draw fresh water on demand. All you have to do is keep water in the reservoir every few days or so (depending on the size of the reservoir).