Are you feeding live crickets?
These can damage the inside of the fish if they kick and rip the stomach or intestine. It's preferable to freeze the crickets to kill them, then defrost them and feed to the fish.
You can also feed freeze dried foods and live or frozen(but defrosted) foods like brineshrimp, bloodworms and prawn/ shrimp.
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Any chance of a picture and short 30 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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What was the ammonia, nitrite, nitrate and pH of the tank water?
How long has the tank been set up for?
Do a 75% water change and gravel clean the substrate every day for a week or until we work out what is going on.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
