Best way to plumb in two water tanks?

[Kelpie]

The new-to-us boat has a moulded-in water tank on either side. Some of the plumbing is currently disconnected, but I believe the original setup was: deck filler to stb, then a c.1.25" pipe connecting the two tanks, and the outlets to the pumps coming from the port tank. I was surprised to see no sign of any sort of breather, but it could possibly be hiding somewhere.

Should there be a breather involved?
Do I need to keep such a large diameter pipe between the tanks? Do I really want all the water to be able to slosh down to the 'downhill' side every time I tack...?

Also, I quite like the idea of keeping two separate systems. I'd be able to fill one from less reputable sources whilst keeping the other 'clean'. Not sure how best to go about this.

 

[garvellachs]

With two tanks port and starboard, our set up is to have a filler to the port tank, a hose and stop valve joining the two tanks low down, and a feed from both (low down) to a 3 way valve so that the pump can draw from either. Works well.

 

[NormanS]

I have a port tank, and a starboard tank. Each has a valve connected to a cross-connection pipe. the cross-connection pipe, which also has a sight glass teed off it, has a connection to the FW pump. The deck filler feeds into the port tank. Both tanks have breathers. I take water out of one tank at a time, and the other one is shut off. You certainly don't want to have water going from one tank to the other, every time you tack.

 

[simonfraser]

Water ballast ?