dead goldfish

Question
I was asked to look after my in-laws 4 goldfish whilst they went on holiday. They have all died. 36hrs after having them we noticed they were lethargic and covered in white spots and were all 'furry'. We totally replaced the water, washed the gravel (just in tap water) and added some 'disease safe' goldfish treatment (made by interpet can't see ingredients on bottle/leaflet), the next day 3 were dead and the last one died 4 days later. I have several questions if thats ok. First the details: The tank is 12" x 6" x 6", has a filter pump, gravel, plastic plants(!), they had 2 fish initially and introduced 2 several weeks later, they had a total water change when they came to us, they were fed flakes 4 pinches twice daily. Im guessing from articles ive read that they were fed too much and changing all the water isnt ideal, could this have stressed them?. Could the new fish have introduced a disease? Could the disease treatment have killed them? I want to replace the fish, is 4 too many? How do I clean the tank and equipment to make sure the disease is dead?
Thanks.

Answer
You were feeding the fish way too much. They really only need one small pinch of food everyday. The extra food in the tank could have created ammounia and upset the fish makeing them lethargic and probably killed them. Or The new fish introduced could have stressed out the already stressed tank and killed them. 4 fish is a good ammount for that tank. You are going to want to clean about half of the water then add the neccesarry chemicals. Rinse out the vaccuum just in case. When adding new fish let the bag sit in the new tank for a while about 15 minutes. Then cut the bag open and whatch them for a few mintues. Make sure you cycled the tank for an hour or so before adding the new fish.
Good luck!
Corinna