Is high light without CO2 a problem?

[knashash]

when it comes to macros, IME, under high light the PO4 and NO3 produced by the fish are not nearly enough for the plants, you may need to provide these along with K and possibly Ca, Mg).

What if your tap water is already high in all these to begin with?

 

[Blinky]

Then I'd say lucky you, you get ferts straight out of the tap!
My tap water has <30ppm Ca, ~70ppm Mg, no NO3 and no PO4. I'm not sure if any tap water has K, and even so you'd probably want more, AFAIK you want your NO3O4:K ratio to be roughly 10:1:20.