Thanks for both of your comments.
The picture that Lloyd shown is exactly the same as my burundi. So might i say that the heavy dark lateral line is a feature of burundi, espically for those pond raised and under stress. For other fronts/gibs, no such obvious line can be seen except for those variants.
The follow up question is "what does mean by variants?". Is that means geographical variants or inbreeding variants (not wild type). Secondly, how could we keep the fronts/gibs too happy to show off their contrasting six-stripes without the "dark line".
For me, apart from keeping the satisfactory pH, hardness, temp., I put a small mirror (shared by local group) inside the tank. It seems work. They always play/fight with their image
P.S. regular water change is a must.