How hardy are guppys?

I think they are very hardy but because they are always pregnant and in some tanks always being harassed by too many males they can get stressed easily and be more open to disease.
At present I have a 55 gallon tank with approx 20 females and 2 males in. They are all breeding females and in 6 months I have only had 3 deaths and 2 have been in the last week.
 
Daniel I didnt have any photos so have taken some. Im not sure how many you are allowed to post or how big the files are allowed to be so apologies if they are not right.
From the tank photo the guppies seem to be hiding so have provided some close ups of some of them. I do have other photos of the other occupants if people want to see them.

tank.jpg guppymalesm.jpg guppyfemalesm.jpg guppyfemale2sm.jpg
 
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