Someone that I work with gave me half a bucket of instant ocean salt mix from back when he had a reef tank. Most of it is in a big chunk and I don't know how old it is. Does salt ever go bad or is it safe to use?
I think it should be fine as long as it's mixed properly...I wouldn't just dump that clump into the aquarium (even though there are no fish) and wait for it to dissolve. Try putting it in a bucket w/ a powerhead and let it dissolve that way.
“Success is walking from failure to failure with no loss of enthusiasm.”
Never had a fish swim into a powerhead, and I even have a few small mollies in my tank that hang out by my powerhead. If any fish would swim/get sucked into a powerhead, it's a molly. Don't put a stocking over it. It will become FULL of junk an algae quickly.