Vent hose for holding tank

 We are getting a bit of an odor in the boat and last time that happened it was on our Catalina 30 and was from the holding tank vent hose. We had over filled the holding tank and it went up into the vent hose. That vent hose had a loop in it and would hold the gray water. I guess in 1983 Catalina did not think to run the vent hose straight up. In fact there was no clear path to do so. Any way we replaced the vent hose and what a difference. So guess what. We have two holding tank gauges and do not think we over filled the holding tank (and it is 34 gal verses the 18 on the Catalina) it but because the other two hoses going out of the boat are brand new we will replace this one. This one we put in when we bought the boat as replacing the holding tank, toilet and all accessories was our first big project. We have just replaced the vent hose filter but it did not seem to help much.