This tank is horribly overcrowded. You could keep TWO goldfish in tank max
They're dying from the sheer lack of space and raising nitrogen. Doesn't matter how small the goldfish are, but one goldfish should have nothing smaller than a 20 gallon tank, and then 10 for each additional goldfish. Besides, orandas are fancy goldfish, and I'll assume that the other "goldfish" you mentioned are commons/comets, and these shouldn't be kept together.
First step, try to make up for the lack of space by changing water. You need a water test, test pH, ammonia, nitrite and nitrate every single day.
Your fish are being attacked by ich, white spot. It's a parasite that live under their skin and feed on their blood. You need to treat the tank with salt for at least a week, possibly longer if you want to make sure the pathogen is dead and gone. Ich is among the most common and most annoying fish disease, usually contracted in crowded tanks or under-maintained systems. (Pet shops usually spread this disease if you don't quarantine whatever you bought from there.) You want the salt to be a 0.3% concentration. Dissolve the correct amount of salt in a little cup of tank water and pour it in by the filter outtake. I think that would be roughly a tablespoon per gallon of water