Goldfish - Swollen Gill

Question
Hi Jaymie,
I have a small young goldfish in a 12 gallon tank.  It's an Eclipse tank with a bio-wheel.  The tank is aout 7 months old so it probably has cycled with good quality bacteria now.  The goldfish has a swollen red gill on one side and a partial red gill on the other side.  I've noticed a decrease in activity as well, but still eating.

At Petsmart I saw the following 2 treatments:

Mardel - Maracyn Two Active Ingredient per 10 gal. is 20 mg Minocycline first dose and then 10 mg each consecutive dose

AND

API - Aquarium Pharmaceuticals - T.C. Tetracycline Active Ingredient is 500 mg Tetracycline Hydrochloride per 10 gal.

The active ingredients are different.  
Please tell me what you recommend.
Thank you, Heidi

Answer
Heidi,
It sounds like your fish is suffering from ammonia and/or nitrite poisoning. When you test your water weekly, what are the levels of ammonia and nitrite? Your goldfish will outgrow that tank very quickly. The minimum size tank for one goldfish is 20 gallons with a filter made for a larger tank, because they are very heavy waste producers. Every fish keeper needs to have a liquid water testing kit to maintain the right levels of ammonia, nitrite, nitrate, and Ph levels. Do a 50% water change now, replacing it with clean conditioned water. Then test everyday and do a 25% water change until the ammonia and nitrites are at zero. Since there is no other symptoms, there is no need to use medication at this time. We never use medication unless we know exactly whats wrong and we know that using the medication will heal the problem. Doing the water changes is the fastest, easiest, and safest way to bring the levels down.