the fins on my gold fish are startting to drop to their body

Question
hi there,
i understand that if you do not want to answer a goldfish question....... however i cant find some one who is able to?
I Have aprox. 20 liters of a  tank  and in the tank their is 1 aprox 5inch gold fish ( the one in question) 1 3 inch gold fish and  3 little guppy type fish.. i just had one of my gold fish pass away from the same type of thing.. ( 2 days ago) his fins started to drop and then he practically stopped moving.. i was told to quarantine him and when i did with in 24 hrs it had died.. i just quarantined the goldfish because the tip of his tale fin is dropping .. i am wondering is there something that can be done, what is causing this and or is this a pointless journey and are all the fish in my tank just slowly going to just pass away..... more info ... we had left for several days and while we were gone he the charcoal bag should have been changed when we came back the tank was an awful yellow so we had to get them a new one (obviously ) and then we also added a white carbon filter ( no one had told me that goldfish urine contains more ammonia then other fishy's anyway if you could help it would be greatly appreciated.. and if not i understand many people do not want to deal with goldfish questions.. but thank you for your time
Sophia

Answer
Sophia,                                                      I am soooo sorry about your loss. You have to much fishy and to little water.As you have learned these guys are notoriously dirty , but beautiful little buggers. Do a good water change for the big guy thats left and add a little epsom salt to the tank. I teaspoon per your tank. Also give him a frozen pea... squeezed between your fingers once a week to prevent swim bladder infection. Also.... get a veggie clip from your local pet shop and slice an orange kinda thin , take half the rind off they love it . Great for vitamin C........Good Luck , Tina