The yellowing " from the edge in" followed by the sudden mortality is making me wonder (strongly) about a calcium def.

You pretty much always want to use Calmag w/ PBP, unless your water is signifigantly Ca hard.

Drop the superthrive, too- it'll delay finish.

I would have a look at one of the troubleshooting guides- don't have links handy, but googling " jackerspackle" will get you to the old OG version.

I don't think that your problem is N.