Rusty water in my Diesel fuel 220D

[Kpmurphy]

1970 220D
I put on a water fuel separator about a month ago because my friend Pierre said I had water in my fuel tank.
I get my diesel at the same place just about every time, it is a new truck stop that just about every uses in my area.
I use the car about 50% of the time, I also have a 300SD 1980.
I have not seen any water in the 300SD.
From the picture you can tell I have a rusty tank.
I wonder why I get water in the 220D and not the 300SD?
Kevin