Potassium chloride cannot be used with all water softeners. Almost all or all newer models can use it though.
I have a water softener on my house and because of that, and a high amount of dissolved CO2, I age my water. You could always look into some 55 gallon barrels to age your water in, or a 275-330 gallon tote. I will be getting a tote sometime this summer to use with my 220.
I keep fire eels, catfish, glofish, clown pleco, and goldfish with this water, and all have been doing very well with large 80-90% weekly water changes.