2 ways. First is the aireation (spelling?) method which recirculates the water in the well adding oxygen via sprayers into the water. For this you would have a pump mounted in the tank with a discharge bar with lots of small holes in it. The pump sucks up the wate and puts it back via the bar with the holes, kinda like a fishtank. Second way is sea water in, sea water out. Here you have a pump mounted to the bottom of your boat with a sealed thru hull fitting it draws the water in from and pumps it into the livewell. The water will rise to a certin level and be directed overboard thru a sealed thru hull fitting. This way keeps constant "fresh" water in the tank.
Fair winds and Following seas