(This is a repeat of an earlier post with exact configuration info.) OK, I had the time to do a thorough system check on the Porta-potty on our '77 h27. There are two hoses going into the holding tank. A pump-out hose that leads to the standard port on the deck, and an air vent that rises at least 3 feet to a small fixture in the side just below the toe rail. The capacity is small, but we do sometimes use it overnight when we are not in a marina. We do pump out at least every other day (usually daily). Back to my original question: What do you recommend (if anything) that we add to the holding tank between pump-outs? Thanks, David Lady Lillie