Because I was told to use it every wcWhy are you using prime with well water? Prime removes chlorine and/or chloramine. It can also be used to detoxify ammonia, nitrite and nitrate for short periods of time ... basically giving you long enough to perform a water change within 24 hrs.
Using prime when you perform a wc isn't going to cause these problems. Have you checked the pH and GH of your water? Well water is most often times very hard and, although I've never kept bichir, I understand they do well in softer water.
Sorry to hear about the fish you've lost.
If the pbb dies I am leaving fish keeping for awhileNot the pbb!! Wish you lived closer so I could help you man smh.