Are you running the aerator 24/7 even when no one is aboard? That's essential to the success of aerators 'cuz without added air the tank contents become anaerobic, so the aerator has to spend the first hour or more after you turn it on restoring an aerobic environment.
Air needs to be distributed all the way across the tank...an aerator "bar" limits the distribution unless you can run some "perforated" tubing off it.
Replacing the "vent" thru-hull with an open bulkhead fitting should help, plus it allows you to put a hose nozzle against it to backflush the vent line every time you wash the boat and/or pump out. Anything that's tried to set up housekeeping in it will just get a water slide ride into the tank. However, you would not be able to do this if there's a vent filter in the vent line because vent filters are immediately "toast" if they get wet...the charcoal--even granular--will swell and block the vent line.
--Peggie
The aerator has been running 24/7 for about 2 months since install. The aerator "bar" is an L shaped tube going into the tank. It goes straight down starting in one corner (at the opposite end from the vent), then turns 90* and runs across the tank for about 18" (angled forward and slightly diagonally across the tank) with perforations along the horizontal part (which is about an inch above the tank bottom). I've confirmed it's bubbling nicely along the whole length of the tube. The tank is 51" long, so that's why I'm thinking I may need to add a second aerator tube towards the other end of the tank to get air distributed through more of the tank and so the bubbling agitates more of the tank contents. The current bubbling is pretty vigorous, as it's visible through the tank sides with a flashlight and you can hear it bubbling as soon as the engine room hatch is opened.
The vent is a 1" vent hose (about 7 feet long) to an open mushroom thru hull (no filter). There's definitely a slight flow of air out of the vent from the aerator pumping into the tank and no issues with flushing or pumpout, so the vent shouldn't be an issue.
What I find interesting is that the waste level in the tank seems to have no effect on smell, and it's consistently worse after agitation, then settles back down. The steady state doesn't smell anaerobic (nor is it a particularly offensive smell). I think I might be the only person who actually notices it, although others have definitely noticed the smell after the tank has been agitated.
Adding a heavy dose of KO to the tank does help the smell a little bit, but it's not a drastic improvement over nothing. I've also tried adding sodium percarbonate to see if more oxygen helps, and it has basically no effect on the steady state smell, although it does help settle the smell down a bit faster after the tank has been stirred up. That makes me think the bit of stuff caked onto the tank walls may be contributing to the issue (there's no significant sludge at the bottom of the tank).
Any ideas to break up the junk on the tank walls would be appreciated, especially if I can avoid having to make multiple trips to the pumpout dock (and pay for multiple pumpouts) to be able to do the full OC Diver style tank cleaning. I've seen mention on RV sites of using Dawn dish soap to break up the fats in tank deposits and help them dissolve, but I'm not sure how effective it actually is (or if adding enough would cause a serious tank foaming problem).
