araguia has a larger body structure and alot less red on them
where as tapajos is a more slender fish with more red on there heads
It's always darkest before dawn ... so if you're going to steal some rocks
from your neighbours garden for your display tank, that's the time to do it.
Easiest way to distinguish is the lack of a black horizontal "stress" band in the Tapajos. Some Tapa's appear to get orange gill plates too, which I haven't seen in Araguaia.
Spot size, redness of head, body shape etc can be very misleading as there is quite some variation within species, age, sex and level of dominance.