Do you know your water params? Normal is a pretty vague word. Also, I would recommend getting some simple decor, if you don't have any, which I don't know if you do since you haven't really answered the question yet.
At least some driftwood towards the center of the tank that the bichirs can hide under or behind. They feel secure when they can't see outside the tank.
Just get a piece of driftwood with no holes. Also, unless it's really sharp, I wouldn't worry about your fish getting hurt. I've had my polys with a normal piece of driftwood for a while now, and they've been fine.
Always make sure your driftwood doens't have weird small holes that a fish could get hurt in.
As you know bichirs are clumbsy, but drriftwood is soft when it's water logged, if you buy a pieces with sharp points or edges, you can always take a file to it, or saw off the sharp points.