KH rebounds like it's buffered?

[Philosophos]

So I've been dumping in nothing but pure, perfect RO water at 0kh in to the aquarium. I buffer it back up for osmotic pressure and plant nutrients, plus some trace. For some reason, the KH always tests 3.0-4.0. I'm looking for something more like a 2.0 for egg viability on apistos.

The substrate is older eco-complete. The only brand name fertilizer going in is Seachem's flourish trace. My own ferts are a blend of:

K2SO4
KNO3
KH2PO4
CaSO4
MgSO4
10% iron chelate

Any ideas on what could be doing this?

-Philosophos

[barmby]

You may want to check what is in the KH up powder typically. I think it is the Calcium and substrate

[Philosophos]

I haven't used anything to purposely increase KH outside of some NaHCO3 once, and I've changed the water enough that this amount should be diluted in to negligibility by now. Would you happen to know if there's CaCO3 or NaHCO3 in eco-complete? I wouldn't think that CaSO4 would increase carbonate hardness, given that alkalinity is a measure of HCO3; carbon not calcium.

-Philosophos