My 220g is empty and debating on what to fill it up with. I am thinking of puting 8-10 pirayas. Or possibly a mix tank with 3 of each pygo. Another option is a tank full of ternetzi. What do you guys think.
My personal preference is a one-species tank, because, like BDKing57 said, it's more natural (the different pygo-species do not share the same habitat in the wild).
You could add a lot of other Amazonian, non-piranha fish to a tank that large, though (like different catfish, tetras etc.), when you decide to set it up as natural looking as possible, with lots and lots of live plants, drift/bog wood, rocks, hiding places etc.
Depends on what you like most...if its that large of a tank then a schoal seems to be the best use but you could get a very large rhom and drop him in there and let him have some fun. I personally don't think I would end up mixing pygo's...even though they're mostly shoaling fish I think there would be a certain discomfert in each species but I don't know this for sure. Other's may have success...I guess you would have to introduce them at the same time for best success rate. I don't think I would mix Piraya and any other pygo ever though.