I do supplement CO2 but it's not extremely high - ~6 bps on a 120 gallon tank. I know some plants like Vallisneria can directly use bicarbonate, but wasn't sure if plants would still use carbonates in the presence of CO2. Do bacteria use carbonates at all? I know Aquasoil binds some carbonates but presumably this doesn't last forever...
Plants will preferentially use CO2 over carbonates. Yes, bacteria will use carbonates as a carbon source but it's essentially not worth calculating. It's quite small.
Are you adding any other sources of potassium such as KNO3? The reason I ask is that 3.6 ppm of potassium per day is higher than EI. EI only reccomends 3.2 ppm of TOTAL K. That includes other sources such as KNO3 and KH2PO4.
As far as alkalinity for K2CO3 versus KHCO3 they are the same when targeting potassium. Bicarbonate (KHCO3) is twice as dilute as carbonate. The fact that K2CO3 has exaclty twice the amount of potassium makes the alkalinity equal for both when targeting potassium.
Here is the dose targeting ALL of the potassium for EI using KHCO3. Mind you, if your'e adding other sources you can reduce this dose.
To raise K 3.2 ppm in your 120 gallon tank you will need to add 3.72 gm of KHCO3, approximately 3 1/2 teaspoons
That dose will provide the following,
dKH 0.23 degrees
HCO3 5.0 ppm
K 3.2 ppm
That increase in KH should not really impact most tank inhabitants. Very sensitive creatures? Maybe.