It's a good question Mr FtS. It worried me for a day or two. Here is the answer ( Google is my friend )
"From the gills, the deoxygenated water passes out the operculum, and the oxygenated blood joins the circulatory system. Goldfish, unlike most fish found in home aquariums, can survive in a non-aerated fish bowl because goldfish spend the majority of their time at the surface, where the oxygen content is highest due to the contact of the water with the atmosphere."
So in a stagnant environment there is more oxygen in the top 18" ... which will accelerate the oxidisation of steel.
"Some fish require more oxygen than others. This helps to explain why some fish thrive in specific habitats. For example, trout prefer northern streams because the cool water of the streams tends to retain dissolved oxygen, and the active trout need the extra oxygen. Carp, on the other hand, are sluggish and do not need as much oxygen, which is why carp can thrive in warm, relatively stagnant ponds, such as ornamental ponds."
I still subscribe to the farting fish theory - much more fun than all this talk of operculum, secondary lamellea and gills