[shrimpchips]

Utilizing the return flow really isn't the best way to get flow throughout your tank - it really should only serve to return water from your sump. Flow in tank should be a separate dedicated system, unless we're talking nano tanks.

Any design that places the returns well below the water line will result in draining your tank - just wait, it will happen. Even if you have check valves (which invariably fail) and siphon breaks drilled (which can get covered over and fail) and any other numbers of redundant systems, at the end of the day, I wouldn't have it in my home. $200 - $400 of plumbing and pumps now could (or will) save $10 - 20000 of water damage later.

Edit (Chris posted before me): With the piping going 24" below the water line to reach the bottom of the tank the amount of backflow (siphon) it will create will be very strong (the minimum size of the returns are probably 1/2", more likely 3/4" each). Even with siphon breaks, wouldn't the plumbing not stop draining right away as it takes a while before the amount of air that can be sucked in through the hole through the venturi effect will be sufficient to break the siphon. Either that, or you have a lot of large siphon breaks installed, which would really defeat the purpose of the returns.

Could you drill holes (~5mm diameter) in the eurobrace every 24" or so and use them to zip tie/otherwise secure the piping to the undeside of the eurobrace?