It's a commonly accepted rule that fish need to slowly adjust to
changes in water temperature and this is one of the reasons why bagged
fish are supposed to float for a while before being introduced into a
new body of water. I'm starting to think that this temperature
adjustment rule is a whole lot of bunk. One of the things I've noticed
about pondkeeping is that different areas of a pond can have radically
different temperatures, and that goes double for ponds without any
circulation. Shallow areas without much vegetation are significantly
warmer than areas just 2' in depth, and this situation must be the norm
since one of the tips to trigger faster growth of water lilies is to
lift the pots near the surface because the water is warmer! Even with
these big temperature differences in portions of a pond goldfish and
koi will peacefully cruise from cool areas to warm areas without
showing any concern at all. How can this be if fish are really as
sensitive to temperature changes as we've been lead to believe?

Does it have to do with the fact that a fish can return to water that
it's adjusted to if it finds a new pocket of water that's too
uncomfortable? If so, sometimes it takes a long, long time for them to
become uncomfortable. Is it possible that this rule is mainly for
aquarium fish where the entire body of water is likely to be the exact
same temperature and a newly introduced fish wouldn't be able to find
water of its body temperature?