The Carbon would remove any unknowns from the tap.
I do not think there are any(it's a possibility, remote I'd say), but the CO2 is the more likely culprit.

I'd just tweak the CO2 more, be very critical of that, the nutrients/Ca/Mg etc you can be pretty confidient about.
I use to Keep Riccia as the CO2 indictator, no pearling most of the day, not enough CO2.
Pearling most of the day? Enough.


Regards,
Tom Barr