My 38g Setup (Mega Pics)

[Willito]

Nice shot! Looking at the water level in both boxes, I think you've just corrected the problem yourself. The water level on the outside box has to be lower to create a great gravity affect to produce a stronger siphon. I believe the amount of flow through your over-flow is determined by strength of that siphon. Try lowering it the outbox a couple inches and let me know if it improves.