dying sword plant

My sword plant is in trouble. The leaves are melting/rotting, and the
crown of the plant looks questionable. I have recently started doing
more frequent water changes, I have read that changes in water quality
can cause this. However, I just realized that I misread the label on my
water treater, and I have been dosing at 10x the required amount.
Therefore, I could also be poisoning my plant. The fish and other
plants seem to be ok though.

Should I:
1) Change lots of water
2) Keep changing water at my current rate (10% 2x weekly)
3) Add charcoal to purify
4) Other

The problem with option 3 is that my filter broke a while ago. Since I
have lots of plants, I have just been running with a powerhead. I could
rig something up, but it would be non-trivial.

Thanks.