a 3/4" barb on a waterfall box using ID tubing? I am stumped!
If you're not familiar with fittings, go to the plumbing section of any big box or hardware store and ask your question. Assuming the person helping you knows up from down, (BIG assumption) you should be outta there in a couple minutes, problem solved.
In a pinch, *ahem* I have been known to use short lengths of tubing as temporary adapters but it must be a tight fit. And even then, I wouldn't leave my pond unattended for any length of time unless the connection was submerged. (disclaimer: definitely NOT something I would recommend if dealing with high pressure)
Living 40 miles from the nearest big box store can be a pain at times...and necessity is the mother of invention as they say..