I can't imagine the aquarium salt being too much an issue. I think I'd just rinse it out and be happy.
Personally, I don't keep food in direct contact with the plastic bucket, regardless of it's rating. Just go out and get a bunch of the 5 gallon mylar bags, or smaller ones and put a few bags in each bucket. You then have a very reliable airtight seal, and the buckets add extra seal, protection from the environment, protection from vermin, stackability, and ease of transport.
Really, the $ you save on buckets over time will add up, and that money can pay for the mylar, or the People Chow that goes into it. =)
FREE is better than for cost if the item will do the trick. =)
D