In the lab, researchers normally reduce oxygen levels by blowing bubbles of a
physiologically neutral gas – usually nitrogen – into the water; nitrogen then becomes
much more abundant than oxygen and displaces it as a dissolved gas. One has to be
mindful of confounding variables in experiments of this kind: disease, stress and
strenuous activity can also make fishes – like people – breathe faster. To eliminate
the potential effect of these variables, it is important to work with animals that are
consistently healthy, calm and not fidgety. Moreover, fishes are ectotherms (coldblooded),
which means that warm temperatures can raise their metabolism and
consequently their breathing rate also. Comparisons between oxygen levels must
therefore be done at similar water temperatures.