I normally put videos on YouTube and then post a link on here. We can go to YouTube to see the video.
Ideal temperature for fancy goldfish (blackmoors, orandas, pearlscales) is 18-28C (24C or 75F is my preferred choice).
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The fish in the picture has a milky white area over its eyes and part of its tail. This is normally caused by excess mucous produced by the fish in response to something in the water that is irritating it.
Do you use "Fish Only" buckets for water changes or do you use any bucket from around the house?
Do not smoke, paint, use bug spray, hair spray, perfume, air fresheners or use any aerosols in the room with the tank.
Make sure you don't have any moisturising cream, grease, oil, perfume, soap residue or residue from anti-bacterial hand washes, or anything else on your hands and arms when working in the tank.
Make sure you dechlorinate any new water before it goes into the tank.
Try increasing aeration/ surface turbulence but not too much. you don't want the fish being blown around the tank.
Maybe add some Activate carbon or Highly Activate carbon to the filter (replace each week) and see if that helps. Rinse carbon before use to remove any fine black dust in it.
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What do you normally feed the fish?
You can try raw/ cooked prawn. Remove the head, shell and gut (thin black tube in body) and throw that away. Use a pair of scissors to cut the remaining prawn into small bits and offer 1 piece at a time.
Live aquatic plants should be available to goldfish. Duckweed is a small floating plant that is readily eaten and is suitable for them. If no live plants are available then you can use dark green leafy vegetables (spinach, silverbeet, broccoli, etc), and mushed up peas, pumpkin, zucchini, etc.
Make sure they are free of chemicals and rinse well before feeding.
You can feed them raw or partly cook them to soften them up.
Offer a few bits at a time and remove any uneaten food.
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Perhaps try doing a bigger water change each day (50%) and see if it helps. If there is a chemical in the water the bigger water change might help dilute it, and carbon should help too if that is the issue.
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The fish isn't bent over like I thought and that is a good sign. And if it is still eating that is a plus too. If the fish stops eating, then euthanize.
The quickest method is a blow to the head but that is not easy to do to a pet and can be messy.
Clove Oil as mentioned is relatively easy once you have the clove oil.
You put the fish into a small container of tank water and add 1 drop of clove oil and stir it up. Wait a minute and add another drop. Continue to add 1 drop at a time and stir it up until the fish stops breathing. The fish is not dead at this point but is unconscious. Now you put the container of water (with fish in it) into the freezer and let it freeze. The fish will be asleep while this happens.
But before you kill the fish, try the things above and it might get better.