New tank design graphic ... Tell me what you think...

[mark]

It should work if I'm understanding your last drawing plus making an assumption. Plus a Herbies as not drawing air are silent.

All three of you bulkheads are drilled to the same height, and within the tank you have a 90° pointing up (assumption). The heights in the tank of B and the one on the right (connected to E) are the same, with 1 being higher than the other two. On 1 and Z you have no valve and is the backup.

Something that might take some playing. I was doing some experimenting with a Herbie in a 33g running with no overflow box, just a standpipe coming up in the corner from a 1" bulkhead in bottom of the tank. Was just using a little pump so flow was 230gph. I adjusted the valve so the water stayed at a steady height and ended up I had 2 7/8" of water above the top of the standpipe. Where the playing comes in is adjusting the standpipe so the final water level is where you want it.