Originally Posted by sailingdog
Part of the reason flexible bladder tanks are much worse, odor-wise, than fixed tanks is that they generally don't have any sort of vent installed in them... and they tend to produce more anaerobic, bad-smelling, bacteria than a fixed tank that is properly ventilated.
Sorry, but the above is not true. Flexible bladder tanks most certainly do have vents installed in them. Nauta makes a bladder in particular meant for either diesel or as a holding tank. It has an extra liner compared to regular bladders and also a 5/8 inch vent line. My boat has had a bladder holding tank installed as original equipment (it's finally leaking) that I am about to replace, probably with another bladder because of space limitations. The secret with all holding tanks is to keep them pumped out and flushed with fresh water to the maximum extent possible.