DIY alternatives to check valve?

[jcardona1]

Is this a mass produced tank/overflow or DIY? You can drill a small hole in the elbow of the return pipe just before it enters the tank. here is a pic

Drilling a hole is the easiest (and cheapest) way to break a siphon. Check valve is unnecessary and will impact your return gph.

For redundancy, you can drill 2 holes.

this.