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Have you checked water parameters? Ammonia, Nitrites, Nitrates and PH?
How big are the Koi and original gold fish?
Do you have filtration?
No we have not checked parameters, ammonia, etc. We do have a filter. The Koi are 4 inches and 2 inches in length. The regular goldfish, is about 2 1/2 inches. He is the sick one.
I suspect that the Pet shop did not give you correct information about keeping these Fish. Rule of thumb for gold fish and Koi is a minimum 10 gallons of water per fish. The fact you are putting 3 large fish in a 5 gallon tank is throwing off the water parameters: http://goldfishgazette.wordpress.com/2009/09/12/how-much-water-do-goldfish-need/. The ammonia should be 0, nitrites0, nitrates 20 ppm or less and ph at 7.0. I suggest getting an API master test kit because of the accuracy. When you add a fish the water changes since there is more oil from the fish, less oxygen and more feces. The Chlorine remover is not going to help with the water parameters. I suggest getting at least a 45 gallon tank min and get it started. As the fish grow you will be encountering more problems. Right now you need to set up a hospital tank for the fish and move it in. Use some AmQuel to remove the chloramines and ammonia, from the water. You will need a bacterialogical product like Bio spira or fritzyme #7 to help start the tank and Aquarium salt 1 teaspoon full per gallon to aid with healing and to help the fish. You will need to do 25% water changes in both tanks on a regular basis, until you can get the fish doing better. I do not suggest keeping the 3 fish in a 5 gallon tank. You need to continue to check the water parameters on a regular basis because if the ammonia, nitites and nitrates are incorrect, you will find all the fish getting ill. Please let me know if you have further questions. I will be happy to continue with you. Joan