I agree that fighting water chemistry is not worth it as there are hundreds of plants and fish one can choose from that will do well in hard water. Furthermore, most soft water plants and fish in cultivation adapt well to hard water, though breeding some sensitive fish may not be possible due to excessive Ca and Mg in hard water can affect egg fertilization.My first question is why try to soften your water? For most plants soft water is unnecessary. It may be recommended in books but it isn't really a necessity. Over time all plants will soften water but only a select few are affected by anything but the most extreme hardness levels. Peat and other "softening" methods do nothing for the DOC but instead lower pH, which may or may not be helpful. If you intend to grow specific plants that require constant adjustments to your water, I would suggest reconsidering. You fight your water chemistry you're setting yourself up for a long drawn out battle you will most likely lose. Instead, try to find plants that work with your water with little fuss and you and your plants will be much happier longterm.
If you never add more water to the tank, or replace part of the water routinely, I can see the possibility of the plants using up the calcium and magnesium in the water, making it softer. But, I can't recall anyone saying they experienced this.BTW, what is your basis that "Over time all plants will soften water..."? It's wonderful if it is true because I can grow sensitive soft water fish and shrimp by growing plants.