Your tank is probably overstocked as well. It does not sound like it is big enough for the number of fish you are keeping in it. Since goldfish are heavy waste producers compared to regular Tropical fish, they really require about twice the amount of room in a tank than the Tropicals. The general rule that I have heard said for goldfish is 30 gallons for the first fish and 10gallons for each additional fish after that. I am guessing from the size you have given us the capacity of your tank is about 15 gallons or a bit less.

I do want to welcome you to the forum and thank you for joining us and I am so glad you enjoy your fish. Please know that each and everyone of us has been in your place at some time. It is heartbreaking to keep losing pets that we care about and I am so sorry that you are having trouble.