Heat is your major killer of juice in a tank like Protank or Nautilus... so with that much volume, I'd not worry too much about it going bad from coil-heat. Just vape it WAY down before topping it back off, so you're not mixing new with stuff that may have been there through several refills.
On the b&m comment. I actually hated this vapor crap from my first two shops. I found a couple good ones though. Doesn't matter if you want the smallest carto or biggest cloud chasing monster. All normal welcoming people. Keep looking! I'm glad I didn't give up.