In marine tanks, there's not much NO3 export and bioballs do nothing for that, only NH4=>NO2=> NO3 really good and add O2.
Otherwise they really have little function.

Adding a massive skimmer which really adds plenty of O2+ removes a lot more waste products, helps a lot more.

The live rock is a bunch of baloney, yea, you get some critters, but bacteria is easy to grow and that is what and why folks add it.

The very fine pores etc help to NO3=> N2 gas, denitrify the water.
That is why they add that + a skimmer and no Bioballs.
That reduces water changes.
Which is their goal.

Nothing wrong doing this with FW tanks either, I do.
As well as for marine tanks.

I use a sock bag filter, sump etc and a bunch of media(larger Zeolite, SeaChem denitrate etc etc).

Same type of concept.
No skimmer though.

The main issue: sealing up the splashing area at the bag filter location so all the CO2 gas is not lost. Same with bioball sections. Plug all the holes up so it does not degas, but rather, goes back into solution.

Get some good CO2 going, some K2SO4, you should knock the NO3 down pretty quick.

I'd not rely on readings from 3 test kits, rather, use the reference solutions only.
I have some nice equipment, a 3000$ spect, but I still use a ref calibration set to make sure.

Why use a test method when you are not sure about it?
I can look and do a water change easier than making such guesses.

Either do the calibration, or do not use the kit.
There's no 3rd choice there.
Some seem to think so.

I suppose if the method had shown 5-10X that it's as accurate as you feel needed for your purposes, then sure.

Lamotte fit that bill quite well.
Same for Hach.

One thing if the NO3 suddenly slow down the uptake rates, do these things:

Really check the CO2.
Clean the filter, clean/prune plants, light vacuum, maybe even uproot some sections and deep vac if it's been 1-2 years since.
Big water changes, clean everything 2x a week for 2-3 weeks and see.

That tends to take care of most issues folks have.

Regards,
Tom Barr