white coat on gold fish in outdoor pond/pot

Question
I emailed earlier today about my fish being sick floating near the top or lying near the bottom, with thin fins...
I just noticed that there is a coating of dull white and a fluffy spot on one, what do i do?
I did a water change of 20litres and it is a 75 litre pot.I had the water checked a couple of days ago and they said it was perfect... I use (was recommended to) malofix, aquarium salt, stress-zyme and water conditioner...?
the one with white seems to be constantly hungry and the not the others...??
sorry my fish are not well, i want them to be happy!

Thank you very much for your time to answer Jaymie...

Answer
Leah,
 Sounds like your fish are suffering from Cotton wool which is a secondary infection from the fin rot. Remove the infected fish, and place him in a hospital tank. Give him a salt water dip. Add 1 teaspoons aquarium salt to 1 gallon of water. This is done in a separate container. Leave him in the water for 20 mins. You need to stay by him so that if a problem arises you can remove him right way. Do this 3 times a day for a few days. Do not add salt to his tank. That will do more damage then good. You can also use Jungle Fungus Eliminator instead of the salt. I have 11 tanks and when they have problems, I cure them with the salt water dip. I rarely need to use any types of medication and it works for me. Let me know how that works, and if after say 36 hours, and the fish is no better, let me know.