Those are very beautiful Silver dollars you have there my friend. You notice the huge change in one month? They will change a lot more as they grow into adulthood & we will have to wait and see how these turn out. What I can tell you is these are not from the genus Metynnis -- more likely Myleus (based on the adipose fin). What I love about these is that they have a VERY unique dorsal fin -- very roundish and they almost seem to connect with the adipose fin. Please do keep us posted!
I think they may be Utiaritichthys sennaebragai ( maybe ?) .. You are right we will have to wait and see. They are voracious eaters and are growing quick. I will send some progression shots as they get bigger.
Do not think it is a red hook..I have had several red hooks in similar size and they had a different lok. These guys will certainly change their color pattern as tey get bigger. They are already more silver than they were when I first got them and have almost doubled in size within a couple of months.