What "start bacteria" did you use? We asked that one before in your previous threads, and we didn't get the answer.
As I recall, your tank is undersized for even one goldfish. On the top of that, it is uncycled.
You shouldn't have had the replacement fish to make it two again after you lost your first one.
Your new replacement fish died in 6 hours, it can only be one thing - shock.
How did you introduce it into your tank?
New fish can't get used to your new water right away. If you dump it in, it dies quickly. Because the new water has different temperature, PH, hardness, etc.
This article talked about the most common reasons for fish dying quickly after sold. One of them is "shock", and there is solution to avoid shock in the article which is the proper way to introduce a new fish into the tank.
Reasons Behind Why Average Fish Die in 3 Weeks After Sold.
I would have stayed away from CopperSafe. There is nothing safe about CopperSafe. It's heavy metal. It is really hard on fish. The tanks had CopperSafe in it would never be safe for certain species of fish who are sensitive to heavy metal.
In fact, you shouldn't jump into conclusion and use medication before you can be sure what was wrong with the fish.
If you can share a clear photo of your sick fish, we might be able to identify the disease or parasite if there is any.
Pet stores like Petsmart and
PETCO will always overstock their fish tanks. It is not economical for them to have only a few fish in each tank. Their fish come and go quickly, so they are not worried about a few fish dying each day. They never planned to keep the same fish for more than a few weeks tops. Don't learn from them if you plan to keep fish properly.
