Java moss kills guppies?

Goldfish produce lots of ammonia and so can stress the guppies. The PetsMart guppies have some strikes against them from Day One. Moving any female guppy just before term can cause premature birth and that usually kills both mother and fry. Java moss ends up in the eddies of the aquarium and anything else dead or inanimate is drawn toward the same area.
 
Your guppies are dying and then getting stuck....
 
What test kit are you using for ammonia, nitrite and nitrate? You state 0ppm nitrate. That's not necessarily a good thing, because this can also mean the tank doesn't have an established nitrogen cycle.

I'd go real light on guppies for a 10g anyway. Like no more than 2-3.
 
I have no problem with java moss; as a matter of fact, it helps my baby guppies & baby red cherry shrimps to have available hiding spots from potential predators.
 
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