UG grows well for me and I've got a ton of N in my tap. Also add a bunch with EI dosing.
The only thing that's ever caused it to melt for me is an abrupt temperature change (extreme - something like going from 55F to 82F in a short period of time) and lighting.
Are you positive it's getting enough lighting from root to tip? When a patch is so thick that light can't penetrate to the base of the plant, it needs to be thinned and replanted.
Is it getting enough flow? UG does best for me when it's constantly moving with flowing water.
I'd ditch the Excel, only do one water change per week and start dosing regularly. That'll lend stability to the tank. As long as you're making sure there's enough flow and lighting, it'll likely turn around for you.