I will mention that in my younger days, my uncle had an ARCO station and I worked for him for a few summers - this was long before self-service arrived. We had barely finished killing off the dinosaurs.
Anyway, I used to have to "stick" the tanks every day. This was a long wooden ruler, and we had a chart that converted tank inches into gallons. We would compare that figure with the counter on the pump. It was supposed to be an early warning system that would show us if the tank was leaking. More importantly, we would smear several inches of water finding paste on the bottom of the stick. This would turn a different color in the presence of water, and tell you how many inches of water you had in the tank.
There was ALWAYS water in the tank. But the tank pickup was located 4 inches or so off the bottom, so an inch or two of water would sit harmlessly in the bottom. When it got too high, a maintenance guy from the company would come and pump the water out with special gear.
The point to this rambling is that when a tanker truck delivered new gas, it would stir this water up, and it would take several minutes after they finished for the water to settle back to the bottom.
For that reason, I never stop for fuel if I see a tanker delivering fuel to the station.