First thing you should do is flush.
If you have a freshwater aquarium, flush with the aquarium water.
The aquarium water has a decent Ph and has been already dechlorinated.
If not, Buy some water at the store and adjust the Ph.
Whatever you use, just don't use Tap water.
After a few days, check for new growth.
If there is, then your ok to add some nutes. Only half strength at first.
As far as the yellowing.
You can't do anything about that.
But you could foliar feed with Nitrozyme.
Make sure you use a decent wetting agent with it.
My question is: What strain is it? Or is it bagseed?