Does salt go bad with age?
 

Someone up the street has an old bucket of salt I am not sure which brand, it is 2 or 3 years old and she will sell it for cheap, any problems with using salt that is old.

Also does the brand matter that much, if you are buying a used bucket of salt would it matter as long as it is a name brand?

I would think as long as the lid stayed on and no moisture got to the salt it would still be o.k.

You need to think of it this way...IF you buy this salt and use it on your tank, and then everything suddenly dies in your tank, was it worth saving the $15??
Maybe the salt is OK, and maybe nobody added or spilled anything in it over all those years and maybe it's really what it says it is on the bucket....lots of maybes for saving a few bucks.
I would never buy a used bucket of salt that had been opened, but that's me.

Makes sense, thanks.

The next question is, do you need to stick to one brand of salt all the time, will it make a difference if you use one brand of salt and then switch to another?

It depends. I have switched between IO and Kent, but essentially they are the same salt. Many people had terrible experiences switching salt brands this last year, with coral loss or severe bleaching.
I would recommend sticking to one type only, as the whole idea of maintaining a reef is to create stability in the environment. Switching things is not conducive to stability. Would there be any type of logical reason to switch salt brands?

old salt
 

Just my 2 cents worth.
I recently used up a small bag of Corallife Salt, that wasn't too old, but was in an open plastic bag. The salt was only slightly clumpy (ie not too much moisture) but the hydration of some of the buffers etc gave a resulting ph of 7.6. So I second that; use only fresh dry sealed salt (I like the instant ocean buckets). And like Brad said; for the $5/week or less for salt mix why risk it.

Brad,

I have heard of this too, yet it is something I have a hard time understanding especially if the water changes are small, say about 15%.

I have gone back and forth between Kent and IO over the years and have had no problem. But I do weekly 15% water changes. That small amount shouldn't cause a problem, should it, even if switching between IO and some other brand other than Kent?