How long has your tank been up and running with fish in it?
Is only one fish affected by the cut/gash?
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Thanks for the information as it proves to be very important.
Before I can even begin discussing a diagnosis and treatment plan I need to alert you to the fact that your tank is cycling. Listed below is a link which will explain the cycling process. I urge you to read it. Now the only way your tank would not be passing through the cycling process would be if you have been doing large or 100% water changes.
The medicine you were using, Maracyn-Two, did not effect a cure for 2 reasons=
1) assuming the tank is cycling, ammonia and nitrites would counteract any and all positive benefits of any and all medications.
2) The affected fish is suffering from a fungus of which Maracyn-Two isn't the best suited medicine to use. It's normally an excellent product but not specifically designed to treat Fungus.
What I think is best to do here is to bypass the cycling process as this will be the only way to effectively treat and hopefully cure the fish. The way to do this is by changing a large amount of water or all the water in the tank thus ridding the water of any harmful ammonia and nitrite.
Once done add some aquarium salt at a ratio of 1 tablespoon per every 5 gallons of water as the salt will help prevent any secondary infections that might occur.
After the water change and addition of salt begin to treat the fish with either of two products and they are 1) Maracyn (regular) or 2) ,Methylene Blue, click on links below for a view of the products. Both medicines can be easily found at most pet stores. Just follow the directions on the product label for usage/dosage amounts.
Keep in mind that for the treatment to be successful there cannot be any ammonia and/or nitrite in the water and the best way to acheive this is by changing large amounts of water every 3-4 days. Make sure that when doing water changes the water parameters, i.e., temperature, ph ,etc.,etc., do not vary too much.
Once the fish is healed you can then take on the cycling process but keep in mind that the affected fish might be a bit more susceptible to problems during this period of time.
Best wishes and please let me know if you have any questions.
http://www.thetropicaltank.co.uk/cycling.htm
http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+4615+4629+4838&pcatid=4838
http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+4615+4629+4822&pcatid=4822
YOUR TROPICAL FISH RESOURCE
OVER 20 YEARS EXPERIENCE W/TROPICAL FISH---BREEDING, MAINTAINING, CURING
Great!- You're already aware of the cycling process--Very important!
Yes, it would be advisable to change a large amount of water again but remember to not let the water parameters vary too much as this can shock the fish.
There's no need to do extra work by cleaning the gravel and decor. Just changing a large amount of water prior to adding meds/salt is fine. Remember to not overfeed the fish as overfeeding is fuel for ammonia/nitrite growth.
Also keep in mind that the ideal water temperature range for goldfish is 62f-72f degrees.
I can certainly understand your attachment to the fish so if I may make a suggestion it would be to cycle the water using different fish. I'll explain below.
If possible, set up another tank, very basic, no gravel, no decor just the basics, i.e., filter, air, heat. The tank could be 1/2 the size of your current tank. Place some zebrafish or any other hardy fish and use them to cycle this tank.
During the time that the second tank is cycling keep doing the large water changes in the main tank (where the valued goldfish are) and keep the water ammonia/nitrite free.
Once the second tank has cycled you can use the water from that tank and place it in the main tank. Just add 1/2 new water and the filter media from the second tank and you'll have a completely cycled goldfish tank without having the goldfish pass through the life-threatening cycling process.
When I mentioned the filter media from the second tank I meant put the filter media from that tank into the filter of the goldfish tank after the second tank has cycled. The filter media is a magnet for beneficial bacteria.
I have done the above before when I didn't want to risk losing favored or personally valueable fish.
Once again best wishes and thank you for your accept.