Please don't install your Forespar Marelon seacock like this

[KGarwick]

Originally Posted by KGarwick

Older thread, but still a valid issue. I have been told that seawater intake for the flush will magnify the odor problems in the building tank. Accordingly, use fresh water only.

Thoughts?

That ought to say HOLDING TANK, I need to proofread better.

[Quebramar]

FWIW - in my experience odours build up over time as salt water decompose (don’t ask me what happens, I have zero chemistry knowledge). But that’s over time, we have that when away from the boat for a week. On a cruise, seawater is renewed each time we flush and there is no odour whatsoever.
Flushing with fresh water before leaving the boat reduces the phenomenon but not completely. Odours only appear at first flush after the absence and then immediately fades away, so no issue per se.
YMMV