How large is your tank? What was the ratio of water to substrate (how much water and how much substrate?) were you using for your test?
That's not too much of a change. Especially if your KH is that high.
Though, you definitely should not be using pH Down in a healthy tank. Allow your tank to settle where it settles and go from there. Don't use additives like pH Down, which are just temporary chemicals that could end up harming your livestock.
What kind of critters do you want to keep in the tank?