I thought it looked like Pothos too but the label says Phildendron (P.Scandens). I'll try to see if I can find some Devil's Ivy/Photos in the plant nursery near us because I can't find any of those in Lowes.
As for your questions...
-how long had they been in your tank?
They've been in the tank for 2 months.
-was your tank cycled?
Yes, the tank is cycled and has been running for more than 2 years. My usual water parameters are ammonia 0, nitrite 0 and nitrate 20-40 (kind of hard to distinguish between the two). My tap has at least 20ppm nitrates though and that's why I'm really hoping that plants will help keep it lower.
-what is the stock and gallons?
It's a 125 gallon tank with 1x Oscar, 1x BGK, 1x Featherfin catfish, 1x Polypterus P. Palmas, 1x Polypterus P.Beuttikoferi and 5x SDs.
-did you snip the roots an inch or so before placing in filter?
Originally, I didn't. But about a month ago I did snip the roots hoping that it would grow.
-any added light?
No added light. Not sure if it matters but the tank is also in the basement so there's no sunlight. I normally just leave a corner lamp on or open the lights on the tank.
please check if your roots are healthy and growing.I had a similar problem with growth.it got slower and slower gradually.if the roots are healthy it should look white to light brown..IF it's dark brown and kind of mushy then we have a problem in our hands.But i have managed to fix it some what noW and my plant has started to grow back