The migration theory is the most far fetched rediculous theory I have ever heard lol. First off all these fish are juveniles- and juvine fish don't migrate- mature fish migrate for breeding. Secondly what would make a fish die if it couldn't migrate? Depression lol!
It comes down to water and or parasites. Many of these fish are imported with internal parasites that you're not going to see.Combined with a stressful environment the parasite takes over and eventually kill the fish. No one bothers to spend the money for autopsy on a fish but I bet a high percentage of these fish are killed by parasites. The other is water. I bet if you imported the same water the fish was caught in and changed it once a week the fish would be fine. You can't simulate the nutrient rich clean water these fish come from. Our crappy water here with chemicals and basically no nutrients is a death wish for these and many other fast moving water fishes. Your best bet is to get a huge 750 gallon or bigger tank with a single fish and do water changes with aged water. That's my .02
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