Good to hear it's doing better.
As far as plants absorbing nitrate, with aquatic plants it's highly related to available light and co2. Unless you have enough light and co2 for them to adequately respirate they will not absorb many nitrates, or grow very fast (and faster growth generally = more nitrate absorption since they need more nutrients for growth).
That's why aquaponics setups are easier, and cheaper, for nitrate removal. Terrestrial or semi-aquatic plants are much easier to get light to and co2 (since they can get it directly from the air), than fully aquatic plants, thus controlling their growth and nutrient uptake is much easier. You also don't have to worry about competition from algae.
With aquatic, you're generally going to either need co2 injection or supplements (such as flourish excel) and more powerful light to make them absorb nitrates adequately.
You can submerge pothos roots in water (assuming your fish won't chew on them), use plain old dual t8 or t5 shop lights, and get decent nutrient removal, plus it looks nice; though like I said, fruit bearing plants, such as tomatoes are much more effective at it since they uptake a lot of nutrients for growth. I've seen several articles of people who have messy fish like goldfish or tilapia packed into a tank or tub and still can't maintain enough nitrate for plant growth.