Re: K2SO4 vs K2CO3
Okay, let me ask you this question if you believe that, if you add any carbonate such as Ca++ from CaCO3, will it raise the KH or not?
It does not matter if it's K+, Ca++, Mg++, Na+ etc
Plants release H+ all the time for many reasons, not just K+ uptake.
K+ uptake alone is not going to be a major impact on KH nor pH.
Plant use it, but compared to CO2?
KH does not "go away", it can only be split into CO2 and OH by a strong acid (HCL, H2SO4, HNO3 etc), CO2 is a weak acid so all you get is a buffered system. Or by plant uptake under low CO2 conditions........CO2 and KH are buffered system, we can add lots of CO2 and the KH will still be the same. We do that with our tanks. Does your Kh go down when you add CO2?
So we are essentially adding H+'s since the CO2 is a weak acid.
If you add HCL, then the KH will be lowered and split into CO2 and OH.
You can also try these little test(not with your fish!)