This is a routine weekly check to check for the presence of water in the fuel tank.

There is a water scavenge system in the fuel system that draws any water due to condensation from the sump area of the fuel tank and mixes it with the rest of the fuel in sufficiently small quantities that it has little to no effect on the engine. If the scavenge system jet nozzles become clogged then water pools in the sump area of the tank.

If water is detected then the water scavenge jet nozzles are removed and cleaned.

Water is heavier than fuel so it sits on the metal plate at the bottom of the jug and looks like bubbles.