I'm getting a better idea of what's going on here. If this were me, with soft water fish (cories, gourami...what else?) I would use much less, perhaps none. My tap water is 7 ppm GH which is less than half of 1 dGH, and no KH. I don't add anything except plant fertilizers (will come back to this) because I keep soft water fish, most of them wild caught. The pH in some tanks is below 5 (not sure how low, can't measure below 5), in others in the low 6's. I let it do what it wants, because it will at least be stable and consistent. It certainly has been, for 20 + years now. I also have plants, and i do 60% water changes once a week with tap water that fortunately has nothing in it that messes with this.
You also mention shrimp, and most of these need minerals for their exoskeleton. So it might be advisable to use some of the additive, but I would want it less than what they are obviously adding. I would recommend a GH around 4 dGH. The data at the link says it also raises Alkalinity (this is another name for KH) but the number suggests about half what the GH will be. They give 7 dGH and 4 dKH for the recommended level. You might get a container of this product yourself and do it at home, or get the store to do it, whichever. I don't know what they are charging you either way, so that is up to you. But I would try to get the GH/KH lower than what your tests indicate it now is in the RO water.
You could work toward this with the pure RO water you have now, mix it say half/half with mineralized water and see what transpires. Just go slow with any changes in the tank, to avoid shocking the fish. This may well have been the cause, or the fish may have been stressed and ich was present. The end of the flashing suggests you got through it, either way, so you don't want to restart it.
Looking at the data, this would concern me. The product contains nitrate and phosphate. They say that in a moderately planted tank these two nutrients are not necessary, and I totally agree. Nitrate is not appreciated by fish, so the lower the better. Phosphate will be present in fish food and more than you will ever need unless you are running a high-tech setup with bright lighting and diffused CO2. I can't see how much nitrate or phosphate is included, but when the current bottle is gone I would look for something else. I use Seachem's Flourish Comprehensive Supplement for the Planted Aquarium, which has everything except phosphate, oxygen, carbon and hydrogen, all of which will occur naturally in the aquarium. If you get this, make sure it is the Comprehensive; they make several products under the "Flourish" line.