Technically you should be able to grow most aquarium plants without Co2.
Co2 acts more like a booster of sorts along with dosing fertz and lighting to make plants grow faster.
But there are many plants that grow with no dosing or Co2.
Java fern, Anubias types are just a couple that come to mind.
If you are will to add a few root fertz but still no Co2, then you could grow swords and cypts pretty easily.
This all depends on lighting of course.
Thankfully ur piranhas aren’t vegetarian!Should be fine.