I use wonder shell for two reasons that may not apply for you. I just don’t want everyone think I just keep raising the GH. One my fish breed like crazy after the wonder shell and I have very little Ca and most Mg. I would not use potassium carbonate. If you be very careful. The best option is to have fish that will deal with those conditions l. I have said I have simlair water I have discus, Angels, several pleco including “L333 and L134 and several others. I basically have an Amazon system. The more additives the more of a chance of swings. Say with what you have and don’t Chase numbers