Hi Carol. Welcome to the Vega world. Last year I installed a 12 gallon box shaped polyethylene holding tank just aft of the water tank in the bow. It seemed to be the largest that I could fit in the space there without a custom made tank for that space. Originally there had been a rubber tank (not good) there so there was already a pump out pipe up through the hanging locker but no vent pipe. I installed a vent line exiting by the bow. I would have liked a bigger tank but for short cruises it's not too bad. At least. Be interesting to know what other people have done. Iver Tara 2490 [Non-text portions of this message have been removed]