Any chance of a picture and short 20 second video of the fish?
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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How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?
What sort of filter is on the tank?
How often and how do you clean the filter?
How long has the fish been laying on its side?
Have you added anything new (fish, plants, ornaments, etc) to the tank in the 2 weeks before this started?
What medications did you use and when did you use them?
Do you add anything else to the tank (plant fertiliser, buffers, etc)?
What is the water temperature?