It would be siphon feed, so natural gravity forces would pull water from display tank as long as its below the tube coming out of the DT.
The water is feed to it the same way as all sumps or a canister filter works. so 2 feet should work, most set it below or under the tank. the manufacturer direction on the web site doesn't have a recommend distance.
you may already know this...just don't want to mislead what I have here
You would want to use some type of overflow box (or a Drilled tank) with the WetDry filter or any sump- it designed to create the flow, the wet/dry tube in the pic hooks to that, you could shorten it or use PVC pipe, doesn't really matter as long as you have a siphon
BUT If you just put the tube in the tank and your return pump stopped (like a power outage) then you would fill the wet-dry until the siphon broke. which could be a flood.
That why people use a overflow box or a drilled tank with a siphon break?