Some of the fish that i have found the most fragile, have also been some of the most hardy at the same time- for example, i have a panda cory in particular that is quite a few years old, while others i have bought in the past have only last a month.
Same goes with oto's- some of the oto's i have bought in the past barely made it past the first week, while others i have got are going on a year old now.
I think it mostly depends on where the fish came from before they went up for sale, what the conditions in the lfs are and how much time they spend in the lfs's tanks before being bought- also what they are fed and which other fish they are put with is very important too as well as the size of the tank they reside in.
If the lfs looks after the fish well and properly, chances are they won't be so fragile when you acclimatise them to your tank and will have a head start in life in comparison to other fish which havn't been treated with so much care in other lfs's.
With guppys- yes, some are notoriously fragile. I personally find home bred ones are far hardier than shop bought ones (particually ones which are around only 3 months old) which have been transported long distances before ariving at the shop.
For example, i have 2 different lfs i go to when buying livebearers. One lfs always has guppys riddled with deseases and infections (i've seen everything from fish TB to internal parasites), while the other lfs's guppys are almost always in excellent health. I asked the two lfs where they got their guppys from- the lfs which sold guppys in poor health said they got their guppys from a site 8hrs drive away, while the shop whose guppys were in good health said they bred their guppys on site in the back of the store.
Generally speaking though, the fancier the type of guppy, the more weak they tend to be.
When shopping for male guppys, go for ones which are the largest and have more masculine shaped bodies- many highly inbred males grow very small in comparison to the females (although it is normal to have males slightly smaller than the females, there shouldn't be a massive difference in sizes) and have very curvy bodies. Longer tails weight the guppys down and can stress them out in tanks with strong filtration, so try to go for shorter tailed guppys as these are generally stronger.
With female guppys, avoid heavily pregnant or very young ones- the trip from the lfs to your tank can be too stressful for very pregnant females or very young ones. Also look out for and avoid ones which look like they've just given birth (low activity levels, sunken stomaches, drooping tail etc).
Female guppys are more or less constantly pregnant, so its best to go for the largest, most active females which don't look very pregnant.
Having many females in your tank in comparison to the males is very important for both genders in mixed gender guppy tanks- if you have too few females, the males will be practically raping them all the time. If you have too many males, they will be constantly fighting amoungst each other.
So its better to have many females to males (2 females per male is the absolute minimum, more females is better though), as the females can spend more time choosing which males they want to be with and will also have more time to their own, also by having less males the males will fight less amoungst each other, so the tank will be much happier overall

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Having happier guppys will also obviously lower stress levels, so they are more likely to live more fullfilling and stress free lives and ultimately live longer

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