I don't recommend vent filters...
I wish I could put a product on the market that creates the very problem it's sold to solve...that's what a holding tank vent filter does.Oxygen is the key to odor elimination...when organic matter breaks down aerobically (oxygenated), it doesn't stink...it's only when it breaks down anaerobically (without oxygen) that it can create noxious hydrogen sulfide gasses. A filter traps those gasses, but it also prevents any exchange of air through the vent line, causing the tank to become anaerobic. So not only do they create the problem they're sold to solve, but they're also expensive. They only last a year at most...are toast immediately if they get wet--which is almost unavoidable on most sailboats due to heeling...and replacements cost more than $50 apiece.What you need to do is increase ventilation to your tank....straighten out, shorten the vent line...increase the diameter...and use a non-chemical tank product that works WITH nature instead of chemicals that work against nature, and usually only add a chemical odor to the waste odor instead of eliminating it.In fact, just switching to the right product may be all you need to do to solve your odor problem....or maybe, you haven't been using anything? If that's true, no wonder you have odor! So before you resort to any modifications in your system, let's see what can be accomplished by simply using the right kind of tank product. The two best are Odorlos and Raritan K.O. I suspect that Odorlos may be the better choice for your tank...when used as directed, it eliminates odor out the vent on 99% of boats. But before using it, you need to flush out Pump out and THOROUGHLY flush out the tank with plenty of clean water...then use Odorlos as directed in the instructions on the bottle. You might also want to check out the link below...it'll take you to a comprehensive "marine toilets and tanks 101" manual that will teach you how to operate and maintain your system to prevent problems.