system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Do fish get 'out of breath' when they swim really fast?
gpt
Same way humans do - they don't got enough oxygen. Of course, they don't *breathe* though. 

To become out of breath, you must exert your muscles, such as by doing exercise. Muscle use requires energy. This energy can be produced in two different ways - aerobic respiration and anaerobic respiration. Aerobic respiration is what cells do normally because it produces a lot more energy and doesn't poison you. However, it requires oxygen. A cell that doesn't have enough oxygen can only use anaerobic respiration, which produces less energy and creates a toxic byproduct called lactic acid, that builds up in your muscles. 

Lactic acid can be converted to a non-poisonous molecule using oxygen, which is what being out of breath is - after you do some anaerobic respiration, you need even more oxygen than you normally do, because you need the normal oxygen for aerobic respiration *plus* the oxygen needed to clear the lactic acid. This happens to fish too, but where we get oxygen using our lungs by breathing, fish get it using their gills by moving water across them. 

Fish have a particular disadvantage when it comes to being out of breath. If you think about the anatomy of a fish, they're pretty much entirely muscle, right? They also have a ridge of darker meat.  Most of the time, the fish is only using that small ridge of dark muscle, that's optimised for regular use. This is the same kind of muscle we have everywhere. The white meat is only used by the fish when it needs a rapid burst of speed, but it runs out of oxygen faster than the dark meat does, so it builds up lactic acid really fast if it's used for too long.