My Red need help???

My red, now 12", sometimes jerks his body as if something bother his body. He has been doing this for quite sometimes now. I haven't fed it any feeders for a long time. I wonder if he is infected with any bacteria, diseases and if in fact he is infected what should i do? he's eating fine, swim normally. Just that sometimes he jerks his body. I always change water 20% every week and have the temperature around 80. I already add salt and it seem to help a bit. Please advised.