Will my tank flood?

[TheEngineer]

This tank is not going to back-siphon, it's going to DRAIN. (assuming the check valves fail)
(well, it is going to back-siphon as well, but even if the siphon is defeated, the tank is going to drain down to the level of the bulkhead fixture in the overflow. No siphon required.)

You need your supply line to go up past your bulkhead fixture, have a siphon break at the apex of the line, and then back down to the bulkhead. This means you'll probably have to use a smaller diameter piping in order to fit it all within your overflow. A couple of 45s and three 90s should allow you to step-aside your pipe as it comes in the bottom of the tank, and make an inverted U at the top, and then back down and out your bulkhead. A small hole on the bottom surface of the inverted U will keep a continuous (and easy-to-verify clear and open) stream of water that shoots downward, back into your overflow.

The other plan B is to install an overflow bulkhead in your sump that drains into a container that's large enough to hold a lot of water. But that's really not ideal because you'll permanently lose nearly half of your DT volume if there's a power failure. When power is restored, your return pump will run dry unless you have it on a controller that considers sump level.

Simple solution that achieves your goal: Smaller diameter return piping, allowing an inverted U with a siphon break, all neatly contained within your overflow.

Welcome @airmotive! The inverted U will work to stop the water from draining, but I would not trust having a hole being your only line of defense to stop the siphon. You are correct that the tank will "drain" even without the presence of a siphon.

 

[airmotive]

Welcome @airmotive! The inverted U will work to stop the water from draining, but I would not trust having a hole being your only line of defense to stop the siphon. You are correct that the tank will "drain" even without the presence of a siphon.

(Yes, I've been here for a year and a half without posting So I figured I'd start with a subject on which I might have something to offer.)

Agreed. It sucks having to rely on any item that's a single point of failure.
But if we are to work within the OP's design constraints, I think we're left with... a hole.
As far as mechanical devices are concerned, a hole is about as simple as it gets.
No moving parts and easy to verify functionality every day... Just check for flow.
For that matter, make several holes of varying size. If one of the smaller holes begins to show restricted flow, you know it's time for routine maintenance.

That said, plumbing is not a one-and-done item. It requires maintenance too.
(Heck, if you're willing to do the maintenance, you can make check valves a reliable back up. But who wants to disassemble their plumbing every week?)

 

[TheEngineer]

I'm glad you decided to start posting!

 

[AZDesertRat]

I was going to try and stop by Home Depot last night and take some pics but work got in the way.
They sell a 1" threaded black street elbow in the landscape sprinkler section that would screw into the bulkhead facing up, screw a schedule 80 PVC treaded nipple into that facing forward and a threaded or thread x slip or whatever adaption you would need to adapt to your Loc Line into that. You can paint the Sch 80 nipple black with a can of Krylon.
You could do the same thing with all PVC and paint it all black too, a 1" close nipple to get out of the bulkhead, a 1" T x S 90 degree elbow, X" of PVC pipe and a 90 for the top.
It would be fairly compact and once painted black then encrusted with corraline algae wouldn't be very noticable behind your rockwork. Run it up to within an inch or so of the operating level of the tank and your sump should easily contain any backsiphonage before it breaks suction.
Another option would be to plug the bulkhead with a 1" threaded plug painted black and run the return over the top of the overflow like most factory tanks were built for years.

 

[TheEngineer]

I was going to try and stop by Home Depot last night and take some pics but work got in the way.
They sell a 1" threaded black street elbow in the landscape sprinkler section that would screw into the bulkhead facing up, screw a schedule 80 PVC treaded nipple into that facing forward and a threaded or thread x slip or whatever adaption you would need to adapt to your Loc Line into that. You can paint the Sch 80 nipple black with a can of Krylon.
You could do the same thing with all PVC and paint it all black too, a 1" close nipple to get out of the bulkhead, a 1" T x S 90 degree elbow, X" of PVC pipe and a 90 for the top.
It would be fairly compact and once painted black then encrusted with corraline algae wouldn't be very noticable behind your rockwork. Run it up to within an inch or so of the operating level of the tank and your sump should easily contain any backsiphonage before it breaks suction.
Another option would be to plug the bulkhead with a 1" threaded plug painted black and run the return over the top of the overflow like most factory tanks were built for years.

That's my suggestion, more or less, too. I think the OP doesn't like having the plumbing visible. Aside from this, your only other option is to have a larger vessel for the water to flow into should there be backflow from the tank.

 

[Scotty LaBeouf]

Any possibility of a drawing for plan A? Have you seen the pictures of my overflow box on the previous page?

This tank is not going to back-siphon, it's going to DRAIN. (assuming the check valves fail)
(well, it is going to back-siphon as well, but even if the siphon is defeated, the tank is going to drain down to the level of the bulkhead fixture in the overflow. No siphon required.)

You need your supply line to go up past your bulkhead fixture, have a siphon break at the apex of the line, and then back down to the bulkhead. This means you'll probably have to use a smaller diameter piping in order to fit it all within your overflow. A couple of 45s and three 90s should allow you to step-aside your pipe as it comes in the bottom of the tank, and make an inverted U at the top, and then back down and out your bulkhead. A small hole on the bottom surface of the inverted U will keep a continuous (and easy-to-verify clear and open) stream of water that shoots downward, back into your overflow.

The other plan B is to install an overflow bulkhead in your sump that drains into a container that's large enough to hold a lot of water. But that's really not ideal because you'll permanently lose nearly half of your DT volume if there's a power failure. When power is restored, your return pump will run dry unless you have it on a controller that considers sump level.

Simple solution that achieves your goal: Smaller diameter return piping, allowing an inverted U with a siphon break, all neatly contained within your overflow.