how often does the tank get cleaned, or how strong is the filter on it.
we have found that the cleaner the tank, the less the guppies like it.
We cleaned some tanks regularly and the guppies got sick and died off, while some guppies in another tank, that hasn't had a clean or gravel vac since it was setup about 8months ago and the 3 guppies that survived the other tank are thriving and multiplying like crazy.
Hope this helps
Ballistic