"Capt. JG" wrote

Water absorption in diesel? Seems to me that would be a very, very small
issue, given that they don't mix.

But they do. See Rich H post below.

If the bio is stuck to the sides, why would you want to knock it free?

So that it can get into the filters to be removed and not come off at an
inconvienient time. Biocide is just a way of assisting its removal but the
stuff is nasty and tends to dump a lot of stuff into your filter system all
at once if the tank is badly fouled. The mechanical action of slack tanks
gets some of it into the system more gradually; especially with some
assistance from something like StarTron.

Keeping the tank "topped up" to the extent practical in normal operation
usually means that there will be full air and water vapor access to the
entire surface of the fuel. The difference between that and a slack tank is
insignificant (for a nearly straight sided tank) and, as Rich points out,
more fuel volume for the same air/fuel contact area means more water
absorbtion.

--
Roger Long