A lot of things can affect the tone of a bichir. Substrate is the easiest to recognize, water quality is one where a before and after really shows the difference, if they're hungry, the lights are on too long, the lights are off for too long, how stressed they are. A lot of things can affect the tone of a bichir. Some more than others and for some they aren't affected by some of the things I listed. I have seen some of my bichirs get completely faded when I leave town and return and they are hungry, others look the exact same. It really depends on the poly. Almost all of the time substrate will have an affect on your polys, same with both having the lights on or off for too long. And for some stress is a really big one that you will notice.