Sometimes fish are sold too soon,
I really hope yours does well for you.If you are curious about what to do with a too far gone fish this method has worked in the past with both fresh and salt water fish.
I find if my fish are sick (I've had them come in with velvet,severe pop eye, one fish beat the daylights out of another beyond help) this method works for us.
http://www.wisegeek.com/what-is-the-...ize-a-fish.htm
http://www.oscarfishlover.com/helpfu...euthanize-fish