Nick,
From where do you have the data that high GH blocks the uptake of potassium
?
What plants are effected by this ???
PH and GH are not to close to each other, it is the KH that effect the PH.
(Carbonate Hardness)
You can soften your water by filtering with peat or use RO water - but not
knowing your tank specs make also difficult to answer.
I have in my planted tank water are hard:
My PH is 7.2 GH is 12dh - Hard water
Plants seems happy and green!
HTH

Amit

"Nick D" wrote in message
om...
Does anyone have any ideas about how to reduce GH without affecting
the PH? I have too much Calcium and Magnesium in my tap water and
would like to reduce it without compromising the PH levels. My GH
right now is 10deg, which is roughly 180-200ppm. I think this is too
high and it is also affecting my plants as high concentrations of
Magnesium/ GH block the uptake of Potassium (the reason my plants are
yellowing in an otherwise healthy tank.)

Thanks in advance,
Nick D.