The only way to really soften water is to dilute what you have with distilled or reverse osmosis water. These types of water have little or no minerals and you mix them with the harder water to get a more desirable GH
Reverse osmosis (r/o) units come in a range from portable to permanent models that are fixed in place. They have various water wastage ratios like 1:1 ratio which means you waste 1 litre of water for every litre of pure water you make. The waste water has all the minerals and stuff in it that has been removed from the clean water. Lower quality r/o units have a higher ratio with more waste water (2:1 or 3:1) meaning you get 2 or 3 litres of waste water for every 1 litre of pure water.
Distilled water can be expensive to buy but you can make your own if you live in a warm climate.
You can make a solar still, which works well in warm climates but not so well in cold climates. You can use them indoors in a warm room or house. It would give you pure water, no waste water and be free to make pure water, it just requires a bit of sunlight or warmth.
Get a large plastic storage container and put it outside in the sun or in a warm room.
Pour a bucket of water into the storage container.
Put a clean bucket in the middle of the storage container. Have a rock in the bucket to stop it floating around.
Put the lid on the storage container.
Put a rock or small weight on the lid in the middle, so the lid sags above the bucket.
As the sun heats up the container, water will evaporate and condense on the underside of the lid. The water will run towards the centre and drip into the bucket. When the bucket is full of water, you put it into a holding container and put the bucket back in the storage container with another bucket of tap water.
You get pure water with a pH of 7.0, 0 GH
, 0KH and no wasted water, no power used and it's cheap to set up. This water can then be mixed with tap water to lower the GH