"Dave S" wrote in message
...
How do I reduce a KH of 150mg/L to 80-100mg/L?

Use water filtered through peat or use RO water for your water
changes.Alternatively stuff some bogwood in the tank or peat granules in
your filter.

Before you do this, why do you want to do it? Unless you have specific fish
that require it (and most don't) I should leave well alone (!)

I.