Hi and welcome to the forum
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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You need to check the ammonia levels.
You shouldn't change filter materials unless they are falling apart. If you remove or replace the filter media, you get rid of the beneficial filter bacteria and cause water quality problems. Filter materials should be washed/ squeezed out in a bucket of tank water and then put back in the filter.
If you can post a picture of the filter or the brand of filter, we can offer more info on cleaning it.
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Do a 75% water change and gravel clean the substrate every day for the next week or until we work out what is going on.
*NB* Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
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The following link has information about what to do if your fish get sick. It's long and boring but worth knowing.
https://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/