ADK13

Hi everyone, I have a 20 gallon tank tall with a 20 top fin filter(thinking in adding a bigger one soon) my KH = 12 and my PH =7.4. I have 6 guppies, 6 espei rasboras , 5 albyno Cory, 1 hill stream loach and one blue ram, I recently added some tetras but they all died because of my high KH, now I’m thinking in adding some chilli rasboras but I don’t want the same outcome, I don’t want to overstock it too much. Also any suggestions in what filter to get and what KH should I lower my tank to.