Segnal bichir need help

[Az89]

I have a bichir that I have had for a little over a month now hes in a 40 gallon tank. He was doing fine up until this last week i noticed he hasnt been swimming around alot an usually when i feed the other fish he swims up to the top to eat an this week he hasnt. Some days he will stay in the same spot forba long time an sometimes his breathing is kinda of heavy an sometimrs its normal. Im just getting worried, I tested the wAter an everything was good except nitrites were barely showing up about .5ppm an nitrates to but i got the nitrites to 0 an the nitrates almost gone but i know nitrates arent that toxic. I someone can help or tell me if im doin somethin wrong i would appreciate it. Thanks