It isn't as difficult as it might seem. As you increase the amount of water you pump, the siphon will automatically increase the amount of water that is returned. Decrease the pump and the siphon matches.
Things will stay balanced unless you exceed the limit of the siphon's capability. If that happens, you will overflow the display tank. That's a sign you have to throttle back the pump a bit.
And...of course...you have to give a bit of thought to the possibility that something (suicide snail) could cause a sudden change in the flow rate of the siphon. That's where some sort of float switch can save you. The float senses a sudden change in water level and shuts off the return pump before something bad can happen.