A Sweetank works with one vent just fine. The two vent arrangement is for natural air flow through. If you are replacing a tank, I would go with two large vents. If using the existing tank, I would install a Sweetank or make something similar. I put in a Sweetank and it took about 45 minutes to install it. Only hard part was pulling thr trigger to drill a 2” hole in the top of the tank... After 2 hours the stink was gone completely. I did install it when my dock neighbors were gone because the first 2 hours were quite stinky until the aeration was complete.