hm.. have any pic to post on the setups for your fish tank which u keep the guppies???So that everyone can see what wrong setups u made that cause the problem.
One thing very important about buying from LFS. Always put them into a qurantine tank before u actually mix them with your home guppies.
Qurantine tank water consist of 50% of your Fish tank(the one which u keep your home guppies) 50% of it is your LFS water which stores the guppies.
Add in some salt.Don't need to add water conditioner(some said need to add water conditioner. Those waters that u used have already contained water conditioner unless too rich to waste water conditioner its ok).
Keep them for like 1 day in the qurantine before putting it into the main tank with the home guppy.