Thank you so much for taking the time to do this. I get the point of the pump and the 2 U tubes, however, I do not fully understand the function of the airline. Also, is this design fail-proof? Ie. if any pump in the DT or frag tank fail, would anything flood?If you are interested in making something that does not require drilling or hard plumbing the tanks together, here is a simple concept that will accomplish an interconnection.
With the tank tops at the same level, use a basic powerhead to induce (not force) flow through the left "U-tube". Leave a gap between the powerhead outlet and the "U-tube". The right "U-tube" will equalize the tank levels. With an airline, connect the low pressure venturi port of the powerhead to the high point of both "U-tubes" to remove any air to keep syphon.
This is just the basic concept.