kh low suddenly

Hi! We have a 20 gallon tall tank with 4 Glo-fish Danios (3 Electric Green, 1 Sunburst Orange), 6 platies (2 Sunburs Wags, 2 Red Mickey Mouse, 2 Red Wags -- 1 Red Wag is looking poorly, staying on the bottom of the tank), and (long story), a baby Black Lyre-tailed Molly. I had wanted to set it...