Yes, the red fishes are Serpae Tetras, aka here in Brazil as "Mato-Grossos", maybe a homage to the State where is the greatest part of the Pantanal.
Nutrients in water may be low, but as they are in a river stream, I am pretty sure that they never run out. So, this is the results of permanent "water change" with constant CO2 and nutrient levels.
Even for us, from Brazil, this pictures are astonishing. I, myself, never have the opportunity to travel there.