So I've never had Danio's before and in the empty well established 20 gallon I had I decided to get 9 celestial pearl Danio's, the day I bought them my lfs said I didn't need to do anything just float and drop, the first day they were fine second day I had one dead, woke up today and their all gone, all my water parameters are perfect 0 ammonia, 0 nitrite,, 5 nitrate, tds 178, gh 6 kh 4, temp 76.5, the only thing out of their liking was ph mines 7.7, so last night wren I seen them at the surface i did an emergency water change, that didn't help, lfs won't give me any store credit or replacements work hardier fish, after me spending $90 on fish, and $170 in. Other supplies saying it was my fault, I've never in the 11 years I've been doing this have killed 9 fish in a day, can someone give insight to what could have happened and how I could of prevented it.