[hellosailor]

Dave-
Salt water is full of critters and nutrients. So is scummy pond water. Use either one in the head and if there is any sitting in the pot or lines it will of course be an active culture medium and grow stinky things. Most of us use it anyway & try to pump dry, because that's the only practical option.
But if you've got the space, by all means add a day tank to flush with potable water. The safe way to do that, and ensure there is no possible contamination back, is to literally install a small day tank up against the overhead. Fill it once per day (etc) with a galley foot pump or small electric, as you see fit. And then it can gravity drain into the head, to flush it. You may want to "T" that line to the usual raw water intake as well, just in case you're tight on fresh water and want to flush with raw water some day.

 

[hellosailor]

Dave-
If you've ever noticed, the overflow drain on a sink is always located well below the mouth of the faucet. So there's no way water can back up into the supply from a filthy sink or drain, it can't levitate.
If you set up the daytank fill similarly, so the water physically cannot be siphoned back into the supply (i.e. tank supply is higher than the tank air vent or overflow) then you get the same protection. This is the universal gold standard for all big-city sanitation codes.
Do it right, and there are no moving parts and no possible way to get sick.

 

[hellosailor]

Dave, have you looked inside a toilet tank in a home toilet? Are you familiar with the fill valve?

Forget the old ones with the big bulb on a stick, they're obsolete. The new ones use a circular float that comes up around the fill tube, and the whole assembly (tube & valve) is about five bucks. That's all you need for a proper and legal "sanitary break" on a fill.

If you look it over...it allows water to enter and fill, with or without a tank, and it is physically impossible for the water supply to be contaminated with "toilet water". Which, by the way, is also perfectly potable in a toilet TANK, as opposed to the bowl. So you've got a $5 part that has all the fancy engineering done. Just needs a hose and optional cut-off valve to supply it. And not just one, but TWO up-to-code sanitary breaks protecting your water supply.

If your holding tank is located above or equal to your fresh water tanks...that's a bigger threat than plumbing fresh water into your head.(G)