I went to Petco just recently and found that they put a small cup with a lid in the aquarium. Half of the cup was filled with salt and the lid has a small hole on it. Is this good for the goldfish? I wanted to do one too for my 2.5 gallon tank but wasn't sure whether it is good for the fish.

If you put the relative amount of salt for the size of your tank it will certainly help if you have an ill fish. I can't really help you much more except to say make sure it has anti-baking mixture (its amazing what u learn on these forums )

I am sure something else will be able to give you more advice.
Good Luck

Only thing i can think about
when i hear salt being use in an Aquarium is helping
ill fish or killing bugs.
Sorry if im not that mutch help.
If im not mistaken i remember someone off the board
saying that salt can break up your
filtration system thing and you need to recycle after?
(Im not sure about this)

My guess would be they are putting salt in there to reduce the stress of traveling, cramped quarters and who knows what else to fish....