It's doing quite well for me in a soil based tank with good moderate to high light and moderate dosing of glutaraldehyde (carbon source). It's not a fast grower but it's definitely growing and has good color.
I have lots and lots of P. Helferi in several tanks, some with co2, some without. While it definitely grows much better with co2, it is not impossible to grow without. Make sure you have a well balanced fertilizing regimen, and keep an eye on the leaves. I noticed they tend to turn pale/white when they are lacking in nutrients. I've never seen another plant in my tank do this. But the most important thing I've found is to leave it alone. Once you plant it, don't disturb it or try to move it.