Peggie - Forgive me if this is totally out to lunch, but I have just enough information here to be dangerous. From reading your posts its clear that the number one way to reduce holding tank odors is to ensure a sufficient exchange of air to keep the tank oxygenated. My problem is that I have a flexible holding tank (an issue for another day and an increased buget). Since the tank collapses there is no room for air in it and so I assume its very easy to get an anaerobic state. My question is: what happens if I add salt water aquarium products designed to help keep disolved oxygen levels higher? It seems to me that since these products are designed to be safe for reef organisms in an aquarium, that they must be safe for the bacteria in the tank. Kent Marine for example makes a product calle PoliOx that is designed to reduce algae levels by increasing dissolved oxygen, and in turn increasing the effectiveness of the bacteria in the water. My theory would be to use K.O. in my tank, and to feed the resulting colony with PoliOx. I understand that a nice big vent with a big line and lots of gas exchange is a better solution, but its not practical for this boat at this point. When I replace the tank someday, who knows. This is what happens when I can't go sailing and the cable's out so all I have to do is stare at the aquarium. Justin - O'day Onwers' Web