By Rich

Rover has these handy removable inspection ports to allow access to much (but not all) of the inside of the fuel and water tanks:

In the bottom of water tanks you’ll often find growth like this that quickly goes into solution when stirred as seen in the bucket (second photo).

Not what you want to be drinking so it’s a good practice to periodically pump the last drops of the water out of each tank with a hand pump and then wipe down the interior surfaces as best as they can be reached. One water tank down and three to go!