In Czech republic its common to have flow-through aquarium with Rainbow trouts in water treatment plants. If something changes in the water, fishes react even sooner than the monitored indicators reach the alert limits.,(reply>), @spokolokofly many fish display behaviors depending on the types of polutants. You can see fish laboring to breathe, flashing off items in their environment because a pollutant irritates them, etc. Color washes out with things like stress from ph or temp swings, etc, etc. It is quite complex, your average fish. I would assume they use a whole bunch of different ways to observe them, and criteria that go along with it. Just came here to throw my fish knowledge in the mix. It's interesting how much you can learn from observation alone,(reply>), @spokolokofly The fish are mostly under the surveillance of cameras or people, if they die, it is quite likely that the water is very polluted,(reply>), how do they respond?,(reply>), Love from Poland, our close friends ^^