Brass is fine with neutral fresh water. If your water is very acidic or alkaline you should avoid using brass. It definitely shouldn't be used with saltwater.
If I were you I would stick to plastic fittings for your tanks. You can find any type of valve you need in PVC, they're usually cheaper, too.
By the way, look for "full-flow" valves or go up a size and then reduce down. Otherwise the valving will reduce flow.
Thanks I couldnt find where I wrote this. I did end up sticking with plastic, but you konw home-depot selection sucks haha and they had brass in stock but I went else where.