I just started cycling my tank 5 days ago with 2 zebra danios(petco) and I did everything right and yet a day or two later on died. I did a water change to remove some excess waste just to make sure that's not what harmed her. The day after I got 2 more danios(petco) floated them for 15 and netted them so I didn't get any diseased water. I have my heater at 75, a waterfall filter, a small airstone, I washed all substrate and decorations before, good feeding times, good ph, everything good besides the fish health. Please help I'm new to the hobby and I want to keep pursuing it