Looking at the Columbian tetras, the fish appear affected with Melanosarcoma, a malignant growth. Sadly, no treatment is known and to the best of my knowledge, it's 100% fatal. The flagfish almost looks to be healthy. The darkening is on a vertical bar, and may just be a color variation. The only other explanation would be an internal flagellate infestation, treat with Metronidazole, and feed some f/d daphnia to keep the "plumbing" clean.
Please keep us posted with photos as this progresses/regresses, and I hope they pull out of this!
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Nothing good happens fast in an ecosystem.
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