my boat was originally delievered with a 1" vent line to a 40 gallon holding tank. i tried every kind of chemical out there to get the smells under control. after the tank was pumped it would stay neat and tidy smelling for a max of 48 hours. at the end of it all, i installed a filter and the problem went away instantly. i think it is a function of the water temp where we boat, its usually 85 degree water and that seems to influence the bacteria production a lot.