Ok...I think you've given the answers I need.
I couldn't find the MSDS for WM Exterminodor (which means WM is private labelng it), but their description of it as a "chemical based" product that isn't formaldehyde makes it a pretty safe bet that the active ingredient is an equally nasty chemical called quaternary amonium compound. Any toxic chemical kills off the aerobic bacteria along with the anaerobic...so it's working against the ventilation.
Your vent lines are long, but they're straight...and since air is "whistling through them" through the tank, obviously that's compensated by larger diameter.
of your tank is the main problem. Passive ventilation above the surface can only keep tank contents aerobic to a depth
of about 20 inches...below that, tank contents turn anaerobic by necessity...any rocking of the boat will cause odor
at the bottom of the tank to roll toward the surface. Your tank is 32" inches deep, so no way is passive ventilation above the surface alone gonna prevent odor
from occurring once the level in the tank rises above 20-24". In fact, I'd bet real money
that there isn't much odor when the tank is less than half full...and there wouldn't be any below half full if you were using a bio-active (live bacteria) or nitrate tank product.
There ARE solutions other than just keeping your tank below half full...aeration being one of them. The other--unless you're in the Keys--being adding a USCG Type I MSD (onboard treatment device)...the discharge of TREATED waste is legal
in all FL waters except the Keys...the whole Gulf except the Keys and Destin Harbor (except for a few misguided marinas
who think that forcing all boats to hold sewage to be pumped out and sent it to sewage treatment plants that dump it UNtreated int the water
every time there's a heavy rain is the "right thing to do."
If you'd like to spend some time on the phone
discussing all this in more detail that's specific to YOUR boat than is practical in a forum, send me an email
. On behalf of your dockmates I'll be glad to help you sort out the best options that will get rid of your odor completely.