Your situation is more common in small ornamental fish. Because there are no pictures, we can only judge from the description, which basically conforms to the symptoms of "lean fish disease" The typical symptom of lean fish disease is that the abdomen of the fish gradually shrinks, resulting in body deformation. In the end, the fish is as thin as a skeleton, and the fish doesn't eat or eat a little until it dies. This is a stubborn digestive system syndrome. Once the sick fish appear, it should be isolated immediately, and the plants mixed in the tank should be disinfected.
I hope the answer is useful to you, please adopt it.