So I have a 20 g with 2 mollies, 1 pleco, 2 ghost catfish, 2 platy, 1 tetra, 2 guppies, and a swordtail. Most of my fish before, died and had ich. I just got these fish today and did a 50% water change before putting them in. Should I change the sponge filter because my ammonia reading was in stress readings? Or would that cause more harm?