Hi Everyone,
I really appreciate everyone advice and comments.
I think Kit_E_Cat's comments sum it up for me tho, ya just never know when that little bit extra maybe needed.
I reckon I will drain them initially and after that will keep em full.
I farmed all of my life and all our water was in the old galvanised tanks, so the greeblies wont hurt that much.
and yes BEER is GOOD