like the others said the main problem is that your tank is far far too small for even one goldfish! If you want to keep them you are going to have to get a bigger tank, if not then take them back to the shop and get something that can live in that size, such as a betta, although you will need a heater for those. Researching before hand would have stopped the problem, dont rely on what the lfs says as quite often a lot of them have no training in fish keeping.