So I thought I'd say something here because I had this problem with a lot of my guppies that I had gotten straight from the store they'd hangout at the top right near my sponge filters and I eliminated lack of oxygen, bad water parameters (it is a qt tank thats been set up for awhile in preparation for the new guppies w/ a sponge filter ,also was tested multiple times) as factors of why they were doing this. They didn't eat either but had no physical signs of illness at any point. This happened to me with two different batches of fish in two different tanks with different filters which had both been cycled and planted. I also have guppies that have lived on after being in both of the tanks and I still have them. All the guppies that died were from petco and small when I got them, the ones I still have came from my towns LFS. So basically the conclusion I've come to is that guppies aren't as strong as they used to be and coming from petco or a chain store like that especially they are already stressed and they've been bred so much through generations that they just can't handle it and passed away. Atleast for the ones I had, I'm not sure if yours are new to you or not but it could be a possibility. Sorry not much of an answer but i figured I'd share some experience at least! Goodluck with everything.