In future I would suggest a quarantine tank to prevent such problems occurring and further infecting all your other fish. In another post TN tanks commented that the tank Im assuming it is your 135gal or there abouts had been cycled. I wouldn't think it is your tank caused the problem if it is cycled so possibly it is shock from leaving the Petsmart tank.
A general rule I also use is to NOT buy fish from a tank where fish look ill or dead fish are present. If other fish from the store also looked ill I would also suggest not purchasing any fish at all as some places in Australia filter all there water with only one filter if that makes sense... ie - one filter, filtering 5 or more tanks.
In future I would also recommend asking to see the fish you intend to buy eat as well as asking when they received the stock in the tank. The fish you have may have only just arrived.
Sorry to hear about your fish, there is not much in my opinion that you can do other than leave it in the tank if you MUST or remove it
asap and monitor it. If the fish is poisoned I dont think there is anything that can be done... it may well live but wont be anywhere near as active.
When I introduce my fish to my tanks I generally leave these in quarantine then introduce them into the tank slowly. Fisrtly I switch my light of to prevent shock and then let them acclimatize in the
QT water in a bag inside the tank Im introducing too. After five mins or so I add water from the tank to the bag slowly then release the fish after around 20mins from the initial acclimatizing. Other people may have different ways but this works great for me. I leave the light of for the remainder of the day until switching it on the next morning.
Hope this helps
