correct and i would keep a group of these piranha , best in shoals and IMO odd numbers, the rule of thumb as babies thru juvy stage would be 10-15g per fish as adults you would want ample room for free swimming and to avoid cannabilism etc, so 35g min would be advised.
One piranha in a 10g tank would be fine for now, i would set up a tank and let it cycle out then maybe look into other specie ie caribe ternetzi and piraya to add some color to your tank, but def upgrade,just dont rush it.