We set up a 55-gallon community tank with only small tetras, shrimp, and one gold nugget pleco, it planted pretty heavily but we keep losing threadfin rainbows slowly 1 by 1. In our first experience with them, we got 5 and all but 1 died then we got another 5 from a different store hoping it was just a bad batch of fish. There is one male that is from the first batch that has still survived but the new ones keep dying slowly. No other fish species has struggled in this tank, we've got our water tested twice by a local fish store and the water is almost pristine. The pet store guy thought the one male is beating up the younger ones but he said it was unlikely. We've changed their diet a lot knowing they have very small mouths from baby brine, crushed flakes, to the first bite food for babies. The pH is around 6.7, and nitrates, nitrites, and ammonia are all low. WE also keep bamboo shrimp in there and they are very sensitive to water quality issues and they have been fine. Does anyone have any ideas?