It's called a "sewer vent pipe". It's an open pipe a few feet higher than the water level (so draining water won't jet out of the top of it if there's a sudden partial blockage). Hook one in just below the drain valve, with a T fitting - preferably one with a curved entrance from the T pointing in the away-from-pond direction.
There is also a plumbing valve used under sinks in travel trailers, where you sometimes can't run a vent to the top close enough to the sink to do the job properly. It lets the departing water suck air from inside the trailer so the sink trap won't suck out, but won't let water or sewer vapor emerge into the under-sink space. You might put one of these into the underground housing where your valve lives, if you don't want to mar the scenery with a few feet of pipe sticking up from the ground near your pond.
