I have 8 Albino Corydoras and all they do is swim against the wall all day, even when the light is off. I am guessing my tank has great reflection. They almost never go into the middle of the tank; it's always the left or right side of the tank (usually the side of the filter because there's more space).
However I noticed today my fish weren't swimming at all. They were resting on the bottom, or so it seemed. I tested my water at ammonia 0.5 ppm and 40 ppm nitrate and changed it; the water is now 0 ppm ammonia and 5 ppm nitrate and they are all swimming again.
What are other good ways to identify erratic fish behavior due to environmental stress?
Update: I don't think it has to do with the ammonia. Four days after I initially posted this question, I have negligible ammonia (less than 0.25 ppm) and more or less 5 ppm nitrate. They are still exhibiting this behavior. They will also not swim above the highest plant in the tank (recently my banana plant has grown past the Anubia plant and now they are not scared to feed from the surface). Nothing is wrong with my fish; I just wish to inquire about normal fish behavior, in general.