For my RO aquarium I use a small amount of GH booster. Yes you don't need much for soft water fish and shrimp 1 degree GH should be fine. I don't add any KH to my tank. Instread I have a sea shell in my filter. My PH stays close to 7. GH is mainly a measuree of calcium and magensium, Fish, Shrimp, and plants need both. A GH booster like Sachem Equilibrium will provide both plus sulfur which plants and shrimp also need. With GH boosters as fish and plants consume it excess sulfur will also aquimulate in the tank which will push the PH down. The sea shell is mainly solid carbonate KH. It will only dissolve in the water when the PH drops below 7. So when the PH drops below 7 do to excess sulfur the sea shell will dissolve and react with the sulfur keeping the pH at 7. Note many fertilizers will also push the PH down due to sulfur and other ingredients in the fertilizer.
Note I would not assume your RO filter will supply both calcium and magnesium. It may only provide enough of one or the other. You need both. So using a GH booster will insure you have both. GH booster can be added to the tank dry right after a water change.
Also note that animals also needed sodium and iodine and several other elements for proper health. RO tanks tend to be low on these. And No GH booster or fertilizer has these animal specific nutrients. Fish can get these from fish food but shrimp and snails primarily get it from the water. Since RO water lacks these nutrients I would also recommend adding a small amount of sea salt to your aquarium after a water change. Note it must be sea salt and not regular processed table salt. Sea salt has a mix of many nutrients that are good for a tank while regular processed table salt is mainly sodium. For a 200L that works out to 1/8tsp+1/32 tsp.