Any chance of a picture and short 30 second video of the fish, and a picture of the tank?
If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
If the video is too big for this website, post it on YouTube and copy & paste the link here. We can view it at YouTube. If you are using a mobile phone to take the video, have the phone horizontal so the video takes up the entire screen. If you have the phone vertical, you get video in the middle and black on either side.
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What is the GH (general hardness) and KH (carbonate hardness) of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
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What is the temperature of the water?
What do you feed the fish and how often do you feed it?
How long has the tank been set up for?
How often do you do water changes and how much do you change?
Do you gravel clean the tank when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
What sort of filter do you have?
How often and how do you clean the filter?
Have you added anything new (fish, plants, wood, etc) to the tank in the 2 weeks before this started?
Do you add plant fertilisers, supplements or anything else to the tank?
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Wipe the inside of the glass with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day until we work out what is wrong with the fish.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use them.
Increase aeration/ surface turbulence.