Bad over sight snowballs to big problem

[Delphinus]

Here's another idea ... it's not as pretty, but it should work. On the inside of the tank, off the bulkhead, have another T. The straight-through part is your main output, and the T just goes into a little standpipe that goes up to just above the waterline. Cap it off, and drill a tiny hole in it. Voila! There you have your siphon break and you didn't need to mess about with tubing or checkvalves or whatever.