I have 12 dwarf neon rainbows, 8 dwarf chain loaches, and 12 Amano shrimp in a well-established ~60-gallon planted tank. (The tank and filter have been going for ~6 years. The fish are a little new: about 2 weeks ago, I traded my former fish for these and 4 pearl gouramis that are on order.)
About 48 hours ago, I did my first water change with a new hose setup, refilling the tank for the first time--slowly--with water straight from my garden hose while simultaneously dosing with Prime. The rainbows have been hiding in a dark corner ever since, while the loaches and shrimp are acting normally. The rainbows came out to eat today, but went right back to their corner, after 2 weeks of acting normally.
I live in a place with a mild climate, and the temperature of the tank goes down just ~2-4 degrees when I change ~50% of it's water either the old way (buckets) or the new way (a slow garden hose). All water parameters look fine: Ammonia and nitrites=0/undetectable, nitrate=~5ppm, phosphate=~0.5ppm, GH=23°/411ppm, KH=10°/179ppm, pH=~7.5 (those hardness and pH values are normal for Santa Barbara, CA, where I live).
Did I do something to severely stress my fish? They didn't hide at all after bucket-based water changes, but they're still hiding now.