I always thought it strange, that sellers always listed these with tetras...

[Magnum Man]

silver dollars look nothing like a tetra, and if you put them in your pretty tetra tank, they are likely to eat all your pretty... and typically they are much larger...

but socially, they are very tetra like, and while not as diverse as tetras, there are many different species... my mid sized red hooks arrived yesterday, with no fuss, they joined the group... I now have 4 different species ) although my only spotted one, is still in my tetra tank )


I think this will be a particularly great fish to stock in my 250 gallon tank, they get along with everyone, but do require sturdy greenery....

 

[Magnum Man]

the little tigers ( smaller fish at the top ) are big time root eaters, and are showing the new red hooks how to eat them... they are telling me it's time for algae cookies...

 

[Innesfan]

silver dollars look nothing like a tetra, and if you put them in your pretty tetra tank, they are likely to eat all your pretty... and typically they are much larger...

but socially, they are very tetra like, and while not as diverse as tetras, there are many different species...

They are part of 'tetra' diversity not separate from it. Remember, piranha are 'tetras' too, in the context of your comment. They are all characins.

 

[GaryE]

There are a lot of Characiformes we call tetras, after an older name. It has more to do with our apparent fear of other languages (Latin, here) than with the fish themselves.

Characiformes are very diverse, and very cool. Tetra has now been accepted as an English word by many, although originally it was just a group of small fish within the larger group of fish of many sizes. Hey, there are Characiformes the hobby would call tetras that grow well over a metre/yard.

Once any of us ask questions like yours, we see the limits of what an answer based on hobby 'language' can give us. If we want to dig deeper like this (that's a big "if"), then we have to look at some complex family relationships and the vocabulary that can make them clearer. But we can also say there some honking big tetras.