Why is my goldfish's healed cut turning yellow? Should I worry?

Thank you for your question about your goldfish. I see the area that you are referring to in the photo, but it is a bit blurry so I can't see it completely clearly. If you feel that your fish is otherwise doing well - it is staying upright and balanced, it is eating well, the two fish aren't fighting, and it is swimming normally, then I would recommend to monitor for the time being. If any of this changes, then I would recommend taking your fish to a veterinarian who is familiar with fish. You can use the AAFV website to help identify one in your area if you don't know of any. Also, you can contact veterinarians in your area who see other zoological companion animals (such as ferrets, birds, and reptiles) and they, or someone they know, may also be able to examine your fish. I hope this gives you a place to start, and thanks for using Petco Pet Education Center, formerly Petcoach.

Consider placing him in a hospital tank. Maintain pristine water quality. That's the foundation for healing. Perform partial water changes every few days if necessary. Use only aged or conditioned water that is temperature matched to prevent stress or shock. Add StressCoat or similar product to promote a slime coat. Plain aquarium salt (NOT table or Epsom salt) can be used to reduce stress, improve gill function, treat minor infection and aid in recovery. Dissolve 1 tablespoon of salt per 5 gallons of water in a cup or two of tank water and gently pour it into the tank. Treat for 10-14 days then reassess his condition. It will take time for him to heal. He may not fully heal, but it he can eat and swim, he should be fine.

This could be a bacterial or fungal infection. If there are other fish in the tank, trauma cannot be ruled out. if there are other fish, ideally Tiny should be removed to a quarantine or hospital tank for treatment. If not, you can treat Tiny in the main tank. First, confirm adequate water quality. You can use a home test kit or submit a water sample for testing. The local Petco may offer water testing for free or for a small fee. Aim for an absence (0ppm) of ammonia and nitrites with nitrates at 20ppm or less. perform partial water changes, no more than 30% total volume at a time, using conditioned and temperature matched water. Goldfish are cold water fish that do best between 68 and 74 F. The filter should be rated for at least twice the volume of the tank. Plain aquarium salt (NOT table salt) can be used to reduce stress, treat minor infection and aid in recovery. Dissolve 1 tablespoon of salt per 5 gallons in a cup or two of two of tank water and gently pour it in. Treat for 10-14 days then perform water changes to reduce salinity. Only treat the amount of water being replaced. More often than not, pristine water quality will allow recovery of minor infections or trauma. If necessary, you can use a broad spectrum antibiotic or all in one product, such as LifeGuard (Tetra) or All In One Remedy (Marineland), to treat the tank. You can find these products in the Aquarium section of the local Petco or online. Follow label directions carefully and remove the carbon filter insert during treatment.

It could be an infection (bacterial or fungal) or it could be due to trauma from other fish or tank furniture. Confirm proper water quality and chemistry. You can try some plain aquarium salt (NOT table salt or epsom salt) as a general tonic. It may reduce stress, improve gill function, treat minor infection and aid in recovery. Dissolve 1 tablespoon of salt per 5 gallons of water in a cup or two of tank water and gently pour it into the tank. Treat for 10-14 days then reassess the condition. Perform partial water changes to decrease salinity afterwards. Use only aged or conditioned water that is temperature matched to prevent stress or shock. Goldfish are cold water fish and do best between 65 F and 72 F.

Looking at the photos, I agree that it doesn't look like the typical fungal infection. Generally you see multiple lesions and they tend to be fuzzy in appearance. I'm not sure what that is on the fin but it may be benign. I would recommend to call the pet store and tell them what you are seeing. Check to see if any of the other fish have become ill. Abnormalities on the fins can be fungal, bacterial or parasitic. There's also a change of a defect or injury. If the area is not really changing and your fish is acting normal, you may just monitor it. I hope this helps!

If it is fuzzy, it is likely fungal. You can try with a commercial antifungal medication such as API Fungal Cure, methylene blue or Pimafix/Melafix. These can be purchased online or in the aquarium section of most pet stores. Read package directions carefully and remove the carbon insert from the filter before treatment if indicated. Plain aquarium salt (NOT table salt) can be used in conjunction with one of the above treatments to aid healing. Dissolve up to one tablespoon per gallon of water into the tank. Fungal infections can take up to two weeks to resolve with treatment. Maintain proper water parameters (0 ppm ammonia/nitrites, 40ppm or less nitrates and temp of 65 F to 72 F). If possible, remove the affected fish yo a hospital tank during treatment.

Thank you for the picture - I am unable to tell if the tail is just turning transparent, or if portions of it are missing. If it is truly just a color change, and there are no other problems with C.G. or the other fish, it may just be that he is going to become a different color. If the tail has missing portions, that is more likely to be caused by an infectious problem (such as bacteria, fungi, parasites). I would recommend evaluating your water quality parameters, as you eluded to in your question. Then I would monitor the tail, and if there are portions missing, I would recommend having him evaluated by a veterinarian. They can take samples of the area, as well as from the skin and gills to evaluate for parasites and other organisms.