After spending two weeks on our boat this summer, Lori and I realized how important odour control was for our holding tank. Hopefully this info will help you out.
We have tried the RV/boat ‘blue’ chemicals that are sold, however we found the smell of the chemical overpowering and it gave us a headache. So I started putting some of my odour-counteracting enzymes (that we use for dealing with urine, feces, vomit etc. in the carpet cleaning industry) into our holding tank. Lo and behold, the enzymes broke down the bacteria faster and without the ‘chemical’ smell – just a mild, pleasant scent was left! This solution may not be for everyone, but it works well for us.
A good odour-counteracting enzyme can be purchased at most janitorial supply stores and is usually much cheaper, and more concentrated, than the RV/boat ‘blue’ stuff.
Happy Boating and Rv’ing!