Those are not Metynnis fasciatus. Also, they are not Metynnis silver dollars. They are an immature Myleus/Myloplus, but I don’t know which, since as immature specimens, they don’t have the final shape, nor the final coloration.
The base of the adipose fin in these fish is very short, as in Myleus/Myloplus/Mylesinus, and unlike Metynnis, which have a very long base of the adipose.
A lot of large non-piranha Serrasalmids look similar when young, changing quite a bit as they grow.
One should expect these fish not to be very aggresive, but to grow quite large (possibly up to 7-9 inches) and to become somewhat less elongated than they look now. As most Mylesinus dollars, they are probably rheophilic feeding on plants growing on rocks in rapids; that makes them omnivorous, eating mostly plant material, but also including insects, larvae and other items in opportunistic manner. Again, they should not be particularly aggressive but they get massive and can be boisterous.
Those fish are so young that their gender characteristics are not yet apparent. They will be as they grow, with males developing a bi-lobes anal fin, where’s females anal fin will be a larger, straighter hook, possibly more colorful.
Gorgeous fish. Please continue to post images as they grow and hopefully we all can learn exactly what they are. If you don’t mind, who is selling them and what kind of price? Cheers!!
Wanted to add dither fish to my arowana / stingray / goonch tank.
