so, silver dollar fish have been in the hobby forever, yet still pretty hard to identify...

[Magnum Man]

I'm putting together a large aquarium, with mixed silver dollar species... getting a little ahead of myself, and mixing it up with a few in a 45 gallon, that several will transfer to the 250, once it's ready for fish... identifying some of my older fish has been difficult, as most were pet store purchase "common" silver dollars... which I interpret to mean some sort of Metynnis... but there is a huge amount of subspecies out there, without official scientific names, even the wide bar, and narrow bar, which are not Metynnis, and which there is much visible difference carry the same name...

sorry for the poor picture, but the main tank light is out on this tank right now... these two were both sold as common silver dollars, yet the coloration and body shapes are significantly different, at maturity... possible male and female??? or different subspecies ???

I think this one ( maybe a better picture ) shows the one with the mottled coloring on it's body, and more deeply colored finnage... the other beside it in the previous picture has more length to it's body, no body color at all ( totally silver ) with just a touch or red in it's tail

this one has black in it's tail along the red, and also in it's dorsal, and a lot more body color... I get that they all look the same, when they sell them the size of a US quarter coin, at the pet store, but as they mature, one should be able to identify them...

 

[Magnum Man]

this is the longer one, it's longer between the back of the dorsal, and the tail...

 

[GaryE]

I have a friend doing some work on Metynnis, and his language about the task gets more colourful than the fish. They seem to be driving him nuts. A lot seems to revolve around knowing where they were caught - impossible with commercial trade fish.

 

[FranciscoB]

For the genus Metynnis (what you have, and what people often refer to as ‘common’ or ‘regular’, or even smaller silver dollars) it is not difficult, at least among the 4-5 species common in the hobby: altidorsalis, argenteus, lippincotianus, maculatus, and fasciatus. Only quite rarely (at most places never), additional taxa come around. Among those other species, there are still some questions, but actually there are better known (to icthyologists) than it may appear.
For the few species common in the hobby, one needs to become familiar with them, and know what to look for.
What makes difficult is having bad pictures. The pictures you are showing are not very good , so it makes difficult to see your fish.
However, your larger fish (foreground) is a male, ant it appears to be Metynnis altidorsalis. The other fish cannot be assessed without a better image (full fish, not behind the other, in focus).
In addition to their characters, the coloration can change dramatically, depending on mood or conditions.
Tomorrow I will show a few pictures of 4 species from my tanks, males and females.
At this time probably the world’s expert on Metynnis is Dr. Rafaela Ora from Brazil.
I only refer here to the genus Metynnis, because that is what you have. Other genera (Myleus, Myloplus, Mylesinus, etc.) are another story but if of interest, we can talk some about them as well. I mostly have only kept Metynnis (currently 4 species), and a single specimen of a Myloplus.

 

[FranciscoB]

I misspelled the name- Rafaela Ota.

 

[Magnum Man]

as I mentioned, the main tank
light is out right now, so good pictures are challenging...

 

[Magnum Man]

this morning, I'm witnessing "dancing" between the male, and biggest plain sliver female... this 45 gallon, could be a breeder tank, once the 250 gallon is ready for fish, however it's really intended as the quarantine tank for new fish, as I begin to stock the bigger tank, when it's ready for fish

 

[Magnum Man]

new main tank light is up... 3 more to replace...

note the dollars have been eating the roots, between cookie feedings

with the new main tank light... still a lot of work to do on this tank...

 

[Magnum Man]

becoming a tight knit group...