Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
What time of day are you testing the pH?
Add some carbonates until the KH is at least 3 German degrees of hardness.
Potassium bicarbonate and sodium bicarbonate are the 2 most common powdered sources.
Sea shells, limestone sand, coral sand and similar materials are a slow release source of minerals, and in the right quantity may offset a lot of what the plants are removing. These will also raise the GH.
I would try this:
Set up the new water for a water change with potassium bicarbonate to the KH you want.
Keep coral sand, oyster shell grit, aragonite, or other materials in the filter (I use a nylon stocking as a media bag).