identify problem and treatment for problem

Question
my fishes are behaving unusal.one fish have dark redness around one side of the gill and showing very less movement.another fish is also showing very less movement but do not have any spot or mark of redness..
some fishes have black spots around there fins..
and one fish is fading there color...
i have 8 goldfish and 1 mini shark...
i don't known about tank size but it can take 8 to 9 buckets of water...
i want to known that "is it right to use iodinized salt in aquarium?"

Answer
Sakshi,  I am sorry it took us so long to get back to you. You have way to many goldfish in that tank. The idea of goldfish bowls are the most common misconception. These guy's need big water. They are carp. I will be very surprised if you haven't lost your shark and probably some goldie's by now. Did you load this tank right after you set it up? If you did it has not even cycled down yet and there is no ecosystem (bacteria layer in your rock's) to break down there wastes. Goldfish put out more wastes than any other freshwater aquarium fish. You also need good filtration. NO you can not use iodized salt. You can use aquarium salt but, if you want to save some money use Epsom salts.About 1 tsp per ten gallon's. This helps them be able to absorb the water and release the wastes product's easier. This is how they drink. If the tank has crashed don't get discouraged. Let me know the dimension's of your tank since you don't know the gallon's and we will go from there. Good Luck, Tina