Any chance of pictures and a short video of the fish?
It's probably from poor water quality.
Any idea what the ammonia, nitrite, nitrate & pH are?
Do a 75% water change and gravel clean the substrate every day for at least 1 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.
If the well water is clean you can use it but if the well water is contaminated with anything it will make the fish sick.
If the tank is newly set up and doesn't have an established biological filter, that will make the fish sick too.
You need to test the tank water and the well water for ammonia, nitrite, nitrate, pH and GH. If you don't have test kits, take a glass full of each to the pet shop and get them to test the water for you. Write the results (in numbers) down at the time they do the tests. If the shop says the water is fine, ask them what the results are in numbers.
With the GH, ask what the test results are in (eg: ppm or dGH).