Deaths natural or not ?

Hey y'all I have a problem . I am loseing guppys not one a day but close and its not just natures thining the herd either . I have lost a few of my strongest stock as well as a few of the smaller ones . All the while I have a handfull of deformed fry that I didnt have the haert to put down that are doing fine so you tell me .....
It started happening after I added the UV sterilizer to my tank . I mentioned this in another post , I have since removed it due to the deaths but I just dont understand it I was under the impression that the UV would strengthen my tank if anything not weaken it .. I have lost my most valuable and oldest male guppy " sonny" he will be missed for awhile as well we also lost his first female "sher" I know the names are corny but anyway we loved them and they were very healthy . No signs of disease are present in the other fish or them . I opened them up to see if their was dis-coloreation and seen nothing .
Do you think its just maybe old age ? But if so why the others the young ones ? Any ideas would be apreciated IM worried .
And why would the guppies die and the cichlids and bumble bee 's not ? I mean they are a much more sensative fish than the guppys right ?