You have a high-light tank with a ton of CO2, and I suspect what happened is
that you ran out of some nutrient. That's what caused the nitrate spike:
suddenly, your previously fast-growing plants could no longer use the nitrate
because they were short of some other nutrient. It's like an assembly line
that ran out of one part. The other parts start piling up, unused.
You say phosphate is zero; that might be the problem. Plants need phosphate as
much as they need nitrate, and PMDD doesn't provide it. And you say you don't
have many fish; the primary source of phosphate in an aquarium is fish
Most people with high-light tanks eventually have to supplement phosphate, at
least occasionally. Sounds like your time has come.