Q: I have added Potassium. Sometimes plants will show holes in the leaves from K deficiency. There is a thread here from Diana about using 'Salt Substitute' I used the "No Salt" brand from the grocery because it's mainly Potassium Chloride. I don't remember how much was advised in the thread (originally intended for someone reconstituting Reverse Osmosis water, or starting from zero). I have a 75g tank and I added about 1/8 tsp to 2 qts of water and stirred it until it dissolved. Then I added twice the recommended dechlorinator, (enough for a gallon of water), then I added this into the tank. I did it when I topped off to compensate for evaporation. I wasn't making a water change. I figure I'll add a pinch with water changes once in a while or if I see holes in my Hygro leaves again.
For your soft water, you could follow the directions on the dosing sticky on the main NPT page, at the top, if you want to bring it up with chemistry. Adding some crushed coral, sea shell or egg shell is a good solution, too and can help keep your tank stable. You can push it into the substrate if you don't want to look at it. I think that if your GH is 75ppm, then you have about 4.5 German degrees of hardness. Maybe someone can confirm this, the divisor is about 17 for Ca and Mg? You could just add shell.