hi we have bought a new boat with a pump out toilet which smells bad I am replacing the seals but have noticed that where the pipe work comes from the toilet to the tank it has to go up to the top side of the tank .I was told this should be straight or some will stay in the pipe and cause the smell . so should I lift the toilet so that the pipe work out of the toilet is at the same level as the pie work in to the holding tank .also is it worth fitting a carbon filter between tank & vent ,would appreciate any help thanks