How do I identify abnormal or erratic fish behavior?

Question

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.

Answer 1

What are other good ways to identify erratic fish behavior due to environmental stress?

Behaviors vary highly from fish to fish and are almost impossible to generalize, but some abnormal behavior includes:

1. Fishes will show less activity and become lethargic.
2. They'll stop taking food.
3. Will hide most of the time, behind decorations, filters, heaters. They will remain at the same place for extended period of time.
4. Color will fade. Some fishes show stress bars.
5. Fish will swim near to the surface (note: top dwellers always swim near surface. i.e Killifish, Arowana etc)
6. Snails will climb out of the water.
7. Some fish will show white color burns at the edge of their tails and fins.
8. Fish will develop cloudy eye or white patches across their body.

Answer 2

Let's first talk about your tank a little bit.

Ammonia: Must be zero at almost all times. Your tank is having huge difficulties keeping up with bioload it seems, and that can cause a lot of issues, such as gills burning due to ammonia, there being insufficient oxygen to breathe, etc.

Reflection: Fish will not see their reflection in the tank unless you have the lights in the tank turned on and the lights in the room completely off, or if you hold up a mirror to the side of the aquarium.

Now, signs of stress. Biggest signs of stress are fish not swimming as much and not eating. Eating, by far, is the biggest because in my 5 years of fish-keeping, I haven't seen any fish that refused to eat the food it was given. If you choose to ignore those signs, then the next signs will be discoloration, and bumping into things. The last sign before death (if your fish are still alive) will most likely be their scales falling off.