I had thought about the same thing fishytime, which is why I was going to use it for a freshie breeder project. On the other hand, if it was cleaned
really well (all hard water stains out, and maybe even a bleaching too) and the calcium all out of it, then re-filled with water it could be tested for Copper just to make sure there isn't any.
I did this with my 120g as well as with one of my shrimp tanks that had been previously copper treated once upon a time and tested at no copper. The shrimp tank has had shrimp in it and they did not die (good sign). 120g is of course empty but I have my fingers crossed (and toes!
) and it
should be fine at Cu=0
I know it sounds crazy but I have used CLR to clean a really nasty tank and it works! I rinsed the you-know-what out of it with water after, then a vinegar rinse, then another water rinse/soak/rinse and the tank has had freshies in it for the last year or so with no noticeable effects. Not sure how I would feel about testing that on salt (I am not that brave), but something to think about anyway.