return plumbing question

[ElGuappo]

so i am toying with designing a few tanks for the future and am wondering about running the return below the water line.... do back flow preventers work well enough for this or am i going to flood my house????

my current system has the return over the top of the tank and i just have anti syphon holes drilled to keep it from back flowing.....

i am trying to design a gyre tank with about 140G (60x30x18) volume... i would like the return to be in the side panel aswell.... is this too much stress for the side panel? the tank will have a divider running about 75% of the lenth down the middle and a overflow at one end.. this will force the water to flow in a circular pattern.... overflow on one side of the divider and the return on the other... i cannot resize my "paint" drawing of it and i have to leave soon so i have no time to redraw... i will try to add a pic soon..

EDIT: should add this it to be a penninsula style tank..