Tannins aren't going to harm anything except slightly lower your pH. If you don't like the look, I had excellent results from using Purigen in my filters. After a while the tannins will stop leaching out, and you can remove the Purigen. I'm not possitive what effect Purigen has on your fertilizer dosing. I use Flourish and never noticed problems with nutrients getting sucked up.
The sword may have been farm raised emersed (only the roots under water). If this is the case, all the leaves will die and be replaced with suitable underwater leaves. And like I said, the break in period for new plants can sometimes take a while. Keep up with the daily fertilizers, make sure you've got good 6000-7000k lighting, and time will hopefully result in a healthy plant.