Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Don't worry about the plants. The 3 degrees GH is more of a suggested minimum for most plants to be sure they have the Ca and Mg they need.
Set the GH and KH for the livestock.
Seachem Equilibrium and other GH boosters have Ca, Mg, and often potassium, sometimes other minerals. Read the label. Be very careful that the product does not have sodium chloride (salt). You can probably find liquid or dry products. The key to avoiding the shock is to make the change slowly. Especially so great a change. Perhaps bring up the GH a degree every other day. Dissolve the dry products before adding them to the tank. They may cloud the water for a few minutes or a bit longer, but it should clear pretty fast.
Carbonates and bicarbonates are often available in combination with other minerals.
Calcium or magnesium carbonates are limestone and related materials (Coral, shells). They dissolve slowly, creating that slow change you want. You could try a small amount in a bag in the filter, and see if that does it for you. Slow change is just right, but if it seems too slow, then add a bit more. Cautiously. When the water is as soft and acidic as yours these materials will break down very quickly, and might make greater changes in the water than you want. As the parameters get closer to your target these materials dissolve more slowly.
You can get bicarbonates without the calcium in the form of sodium bicarbonate (Baking soda) or potassium bicarbonate.
I think you are going to have a hard time getting the GH and KH that high, yet keep the TDS and pH that low. You might try to see which values are the most critical and aim for that.