What's killing my guppies

[Lupin]

some male guppies have been known to be reasonably fragile because of inbreeding.

Good point. As far as neons are concerned, they're a hit or miss. Some will die faster than most. Guppies..well..plenty are already inbred so that may explain why some die without explanations but I would still not rule out the possibility that the water parameters are whacked at the moment.

 

[Lupin]

sorry, not a hermit crab, a fiddler crab. they like land too, but I didn't find out untill after I added him. He seems so happy.

It is hard to say whether it is really happy or not. Chances are it isn't. You need a part land biotope for it to thrive.

Also, I forgot to mention the betta. I noticed that some of the guppies tails were getting torn up, so I removed him about 3 weeks ago. Could the male deaths just be delayed after effects of betta bullying? I never saw him do it, it was just a theory.

Were the guppies severely stressed and refuse to eat well after the betta bullied them? If not, then water parameters are still not to be ruled out.