Dying goldfish

We recently bought some new goldfish and introduced into our fish tank. And guess what? Apparently, the new fishes brought along with them diseases, either in the water or the fishes themselves. And when we introduced the fishes into the fish tank (water and all), all the original goldfish got infected!!! Argh!!!

All the new fishes have died less one. Of the original, one of them has died too. We tried all means (using general tonic, adding salt) to no avail. The symptoms are those of finrot, so in the end, we went and got a bottle of finrot medicine, and added it to the tank. Hopefully, it helps and the remaining four fishes (three original plus one new one) make it through this ordeal.