Here is a very unscientific observation...In my little fish pond at home, on cooler days when I throw in the fish food the fish come from all directions, under rocks, shallow end, deep end, etc. On hot days they are almost all hanging out by the waterfall...to get the higher oxygen levels caused by the cascade?
Also the larger fish are more likely to be by the waterfall than the tiny ones. Do the larger ones stress quicker and need the higher oxygen levels? Or do they just chase the little ones away?
I've often wondered why they are by the waterfalls when I expected them to be in the deep end where I assumed the water was cooler.