Terrarium questions

[Subrosa]

Not going to work Robert. Your design requires you to, from a physics standpoint, have your cake and eat it too, in that you are trying to combine an open system with a closed system. In a closed system you could indeed get all the air out of your return tube and have water return nicely to the sump. But a waterfall is by definition an open system, which will allow air in which will stop your siphon. I've been installing and maintaining aquaria professionally since the early 90s, and if I had a nickel for every time I've heard someone want to try what you are planning I'd have a few bucks. If I had a nickel for every one I've actually seen, I'd have a nickel, and if I had a nickel for every one that worked long term I'd be broke! The only sensible way to get water from the main tank to the sump is gravity. The only non-sensible way that works is to use a second pump on a float switch, which will one day fail. In your case that would result in a burned out pump in your sump. In the one case I've seen of someone actually doing it, they ended up with about 20 gal of saltwater on their floor. You are correct in that tempered glass can't be drilled. If you want to run a waterfall in an undrilled tank, you will end up putting the pump in the main tank. Save yourself a lot of wasted time and design around it. This is not an opinion, it is physics.