sick goldfish

Question
I have a goldfish in a 30 gal tank.A few days ago he started swimming in circles.Now he sits  at the bottom of the tank,does not eat and he is in a concave shape.He has red lines on his tail.Water was tested at 3 pet stores and I was told the water is good and that it may be the fish.Goldfish is over 6 years.
Please help.
Thanks

Answer
Carl,   First I would medicate your tank Marycin 2. Do not forget to take your carbon out of your filter. It sounds like he has all the signs of a bladder infection that probably made him rub something in the tank and cause an injury that went septic. Do it now. Once they can not hold their upright position the infection is getting worse and he can go quick. I would also add some aquarium salt and stress coat. Just the normal amount of salt not therapeutic does. Though a dip a couple times a day in a therapeutic dose would not hurt. I think that is 1  tsp per five gallons for therapeutic. Hopefully he makes it through all this. After, make sure you feed your goldie a cracked frozen pea every couple days. It is good to keep him from constipating. Also, you can get a veggie clip from your pet shop and give him a slice , rind in, of orange once a week. It is high in vitamin C and he will love you for it. Good Luck, Tina