OK. With those numbers, using lots of Tri-chlor tabs is not a bad thing...High pH, low CYA..Tabs are acidic and release CYA.

I suspect the reason the builder put in the N2 was because he KNEW he had put in too small an SWCG. Now it's broken, probably due to running flat-out at "flank speed". N2 kills algae, which can lower your chlorine demand...but the cost is far more than the cost of bleach. And, when the cartridge is old, they are useless even by the N2 standards.

If you have Trichlor tabs, use them. But you'll need bleach and acid to adjust FC up and pH down.

Carl