died

Hi all, I'm new to this fish keeping lark. We got a tank and added 6 endlers guppies a few days after getting it, as suggested by the pet shop. The fish died within 20 hours. The water tests showed raised nitrate only. We left it for a week before checking the levels again and adding another 6...

So last night I heard a thud, which I'm used to hearing between the black wolf and odoe in the bigger tank, I didn't think anything of it but woke up to a dead del, everyone else is fine, he's not in a tank with anything aggressive, just did a water change. That's my only guess. Was a full 9"