Water Tank

[Ceecee85]

I'm no expert but I'd have thought that the tube is both for letting air out when filling and acting as an overflow but it would also let air in when water was drawn out to use. Therefore, any valve you put on would need to let air flow in both directions- which isn't really a valve then.

[Oldiebut goodie]

Just tie a piece of one of your stockings over it if you are worried about anything getting in. I have never seen any need to cover them though or seen a tank with anything on. On ships all that there is at the top of the vents is a u-bend and a grill to stop rats getting in and that can be a 6 inch bore pipe.
How much dirt are you trying to get in it? You are supposed to eat a peck before you die. Stop panicking and just use it. This over sanitized world that the admen push on to people is unnatural.

[tforturton]

If I read the original post correctly, the bits getting in are just an after-thought.
I think he's more concerned with the reason behind it over-flowing....

[Oldiebut goodie]

The overflow aspect had already been answered.