Many people mistakenly assume that any odor inside the boat, especially in the head, has to come from the holding tank. So they improve the tank ventilation, try different holding tank products, and do everything they can think of to the tank, but the boat still stinks. That's because the odor isn't coming from inside the tank--how could it be unless it's escaping from the tank? If the tank isn't leaking, that can't be happening...the odor has to be originating somewhere else. It may not even have anything to do with the sanitation system (see the article "Boat Odor Isn't Alll in Your Head"), but there are certainly plenty of "head" related sources of odor too. Toilets that use sea water have more odor problems than toilets that use fresh water--whether that water is supplied by the onboard fresh water system or comes out of a lake or river. Sea water, especially coastal sea water, is alive with animal and vegetable organisms...when they become trapped in toilet intake lines they die, decay and stink. If you aren't sure whether that's at least part of your problem, there are a couple of indicators: the odor is concentrated in the head and not all pervasive throughout the cabin--or at least is much worse in the head than in the rest of the boat...and is worse after the boat has been sitting for a few days than after you've flushed the toilet a few times. To be sure, close the seacock and flush the head as dry as possible. Disconnect the hose from the thru-hull and stick it in a bucket of water that's liberally laced with white vinegar. Pump/flush the whole bucketful through the system. You can reconnect the hose to the thru-hull now, but don't open the seacock...use cups of fresh water from the sink to flush the head for the entire weekend. If the odor is gone when you come back to the boat next weekend...bingo. How to cure it depends upon what kind of toilet you have and several other factors, but if you have a manual toilet, a very simple inexpensive cure is described in the article "Intake Odor Cure." However, I do not recommend using the "inline" chemical devices...the chemicals in them can be harmful to rubber and neoprene parts in the toilet. If you have a Lectra/San, the chemicals in them can damage the electrodes. Permeated hoses are a major cause of odor inside the boat, and there's no predicting how long it will take for a hose to permeate--brand new hose permeated on my own boat in less than 3 months...the same brand and type of hose that had been on my previous boat for nearly 7 years without a trace of odor. What causes hose to permeate? Sewage left to stand in it...because 9 people out of 10 stop flushing as soon as the bowl is empty instead of flushing till the bowl contents have been washed all the way through the system...so each flush sits in the discharge line, pushed another foot or two by the next flush and the next... The head discharge hose is an anaerobic environment, so whatever is in it IS gonna putrify and stink. Incomplete flushing also causes another problem: urine crystals that build up in the hose...and as they do, they create a rough surface that traps bits of solids and paper. Over time--and sometimes an incredibly short period of time--the combination of urine crystals, salt water calcium carbonates or hard water minerals, and those trapped bits can reduce a 1.5" diameter hose to less than a 1/2". VacuFlush toilets (I had one on my last two boats, btw) are the worst offdenders when it comes to leaving sewage in the hoses because the suction splatters waste all over the inside of the hose and the vacuum tank and the miniscule amount of flush water that's a major selling point for V/Flush isn't enough to fill the hose to rinse it out. It's the reason SeaLand had no choice but to come up with a "bullet proof" hose--their very expensive "OdorSafe" brand. It's expensive, but worth it IMHO no matter what toilet you have, because I've yet to hear of a single case of odor permeation failure since it's been on the market. So which is really more expensive--$8+/ft once...or $3-5/ft every few years, not to mention what a major pain it is to replace hoses? There is no cure for permeated hose except replacement, only prevention. I've given up trying to convince people to flush enough every time to rinse out the discharge hose...'cuz the inevitable reply is, "But that fills up my holding tank too fast!!" So I had to come up with an alternative, and this does work: once a day--last thing before you go to bed is a good time 'cuz that's when the system is likely to be unused for the longest period of time--close the seacock, flush the head as dry as possible, and flush a couple of quarts of fresh water through the system (VacuFlush owners fill the bowl to the rim and flush). Do that when you close up the boat at the end of the weekend, adding one more step...after you've flushed the fresh water through, follow it with a cup of white vinegar. Don't rinse the vinegar out. NEVER use bleach...nor any chemical bowl cleaners, ammonia, Lysol, pine oil cleaners, solvents or anything containing alcohol or any petroleum products. These not only are harmful to rubber, neoprene and Lectra/San electrodes, they break down hoses. When was the last time you even thought about, much less replaced, the joker valve in your head discharge? After enough paper and solids--just flush water--have been forced through a joker valve, the "duckbill" can no longer close tightly...and if you ignore it long enough, it starts to resemble a large mouth bass. When it can close completely, it blocks odor coming back into the head from the discharge hose...but when it can no longer close completely, there's nothing to stop that odor from rolling out of your toilet. Change the joker valve once a year. Leaky overboard discharge pumps--manual or electric--and y-valves are another source of odor from the sanitation system. Repair, rebuild or replace 'em if necessary. Remember: any odor is always strongest at its source...the only way to eliminate any odor is to find and eliminate the source...anything else just masks it, and it's a never-ending battle. More often than not, finding the source becomes a process of elimination. The bad news is, odor is worse in hot weather...but the good news is, that makes it easier to find the source, remove it, and get rid of the odor permanently. And isn't that what you want to do?