It is interesting. We get south american stuff that goes straight into our water. If you believe everything we are told as hobbyists, one would expect those fish to suffer pH shock at the very least, but cardinals, rummy nose, bleeding hearts and expensive L number plecos seem to adjust well. Perhaps fish are more tolerant than we give them credit for.
With regard to lfs tanks, when I was in the retail trade, all my tanks were individually filtered witg under gravel plates. I looked after them all as if they were home tanks.... water change every couple of weeks (which all the lterature suggested at that time). I got to a point when I knew if a tank needed attention, just by observing slight differences in fish behaviour.
I used to get a lot of hobbyists that had purchased all their tanks and equipment from a local large aquatic outlet in a garden centre, but were getting poor advice and losing fish. Many of these people bought fish from me and would come back and tell me how well they were doing compared to fish from the garden centre.
The only reason I could deduce was that the garden centre run their tanks on large central filtration systems using uv sterilisers to minimise disease pathogens and also zeolite beds to remove ammonia etc. I considered that this meant their fish were likely not exposed to any level of ammonia, nitrite or nitrate and therefore had little tolerence when a hobbyist took them home and placed them in an aquarium where the parameters may fluctuate.
In my experience, many fish will tolerate certain low levels of ammonia, the toxicity of which varies anway, depending on temperature and pH, as long as the increase is relatively gradual. I have found they are even more tolerant of nitrites as it tends to rise even more slowly. I think I have posted before that when I did my first spawning at the fish hatchery I set up at my last job, I had 2 week old fry feeding well and growing in water that measured 8ppm nitrite, as we had not had time to mature the filters prior to starting production.
At my place of work now, we once measured ammonia in one of the marine systems at 1.0ppm...... dangerous / fatal, according to the test kit recmmendations. We didn't lose a single fish and, after adding some extra bottled BB (another bone of contention among hobbyists) the ammonia dropped to zero within a couple of days.