I always say something - i use pets @ home for accessories - NEVER FISH , they very often have a dead fish or one with Ich in their tanks and i always ask for the assistant manager and he gets called down and tell him
I think it's best to get the fish treated so that it doesn't suffer and also so that some unsuspecting customer doesn't go and buy an ill fish and infect their whole tank (if they dont quarentine)
So my advice is ALWAYS speak up (if you feel confident if you dont just mention it to another shopper and they might speak up instead

) as long as you point it out politely they aren't going to mind you doing it - Just think about the poor fishies , i know a lot of people say "they're only fish " but if you have that attitude you shouldn't keep them - IMO
Cheers Sarah xx
Yeay, Pipoodle, (and various others,) you go girl!
As pointed out by rabbut, a busy or uninformed staff may not have noticed or recognized the signs of illness, and in either event it needs to be pointed out in a helpful fashion.
As previously stated, if the fish aren't treated, they'll continue to suffer and possibly die, the disease will spread to more fish, and as well, more people will wind up spending maybe several hundred bucks over the course of a year for meds to treat new fish they may wind up losing.
Nearly every fish I've bought from my regular LFS over about the past year has been sick or had some problem, although previously I'd bought perfectly healthy ones there.
(But it's more stressful for the fish to be transported back and forth, and, assuming I have or can get meds, once I get them home they're mine! so buy stock in Mardel and Sea Chem...)
That LFS generally used to have more knowledgeable people, young girls or not, working there, although they seem now to have mainly young and not very knowledgeable girls, some of whom have had wicked, evil parents who won't let them have even a fish tank.
It was evidently one of these, and a new one, who looked at the betta I'd pointed out as having fin rot, a small (I thought baby but apparently probably stunted, as neither he nor another tiny betta with the same problem bought shortly afterward have appreciably grown) long-rayed (on several remaining ones, anyway and quite a few much longer ones now) crowntail with many varyingly but quite short stubby rays (some eaten off almost right up to his body) coated in actual lumpy black stuff (first I'd seen of that) and informed me that 'he's just like that'.
(I couldn't get him at the time, but he was there, same size, albeit with improved fin-condition, about a month later when I could.
Although he still had short rays with a bit of black stuff, and was still small and hadn't grown at all...
I'd never have guessed he had such extremely lovely colour, although I don't know if his fins'll ever grow out evenly, and both of those little ones are a worry.)
But any experienced person (and there weren't any there at the time) would have treated that betta off the hop - and would never have dismissed something so heartbreaking as 'normal', leaving it to progress until death or a mercy-buy occurred, although in this case, I should think the fish must have been treated at some point in order to have improved a month later.
And if you happen to notice a problem in one fish in a sale tank-full, it may be only because that fish was in a position to allow someone to see this clearly, something which may not often occur in early stages of an outbreak.
Signs of disease which may be quite obvious once they're floating in a bag in your home Q (or worse - other, possibly pre-inhabited) tank may not show at all in the varied schools of fish darting around in sale tanks.
(As an example of potential complications resulting from the purchase of unnoticed sick fish, I found this with some Harlequins I bought some time ago, which still have blackened [but not eaten away] fin-tips/edges and, despite a months Quick Cure treatment and the apparent long-term clearance of this, have just had a [mild] recurrance of what was probably ich but was so enormous I thought it must be another new parasite of some kind.
But the blackened - and at the time shredded - fins and the huge, lumpy white spot on one fish's caudal weren't visible in the store tank, although they hit me like a sack of hammers as soon as I pulled the bag out at home.
And at the time, they didn't even have the right and identifiable shape and looked especially like Halloween cut-outs due to the then-colourless dark-on-light pattern.
So, apart from a week's break from medications, which resulted in a small, lumpy white spot being [again] visible on one fish's tail, these little guys have spent months in isolation, under various mostly chemical treatments which must have cost at least $60-$80 to buy, apart from what I'd had on hand, and yet they continue to become more confident and normal in behaviour, livelier, more colourful, and prettier all the time...
Gotta love the poor little critters - but I'd never have bought them if I'd seen the problems in the store.
Especially as I'd evidently splashed with infected water an adjoining tank, where Graffitti, the most personable and interactive betta I have, suddenly developed horrible black fin rot, and the standard Maracyn and Maracyn 2 combo failed as thoroughly as did a number of other meds tried so far on all affected fish.
I'm hoping Jungle Fungus Eliminator, which I've heard recommended, might do the trick, if I can actually find/get some.
And although I was told by one of the LFS owners that Polyguard, which I'd had on hand, wouldn't help, I tried it on the Harley's, with salt, a week ago, out of desperation and the black on the fins is FINALLY actually [somewhat] diminishing, and so I have the affected bettas on it now as well, and all spare fingers crossed, which explains my typing.
Graffitti was one of my two betta boys which were sold as Half-Moons, and, as far as I can tell, would have been one if not for an outside ray bent the wrong way.
Somehow this makes the current wreckage of his tail even worse, and it's pretty much right up to his body now, having started in one, high spot, under where his beautiful, flowing dorsal fin draped over and hid it...)
If one, or one batch among sale fish, has a disease, the others have also been exposed, and it may well be the final shock of yet another transference into a customers home tank that weakens those other fish enough to allow the problem to flourish.
The lFS I've most often bought fish from has a policy of a 1 week refund on health issues, although one newish girl I spoke to, following an episode of having fish spinning head-down and obviously dying by the time I'd got them home following about a 15 min. drive straight there, attempted to tell me that there were no exchanges/refunds on sick/dead fish, until one of the owners overheard me saying in some disbelief, 'even if they're dying in the bag before you get them home?"
So one staff-person's statement may not reflect store policy.
And, going by my own experience, I'd say that it's better to check out actual policies and treatment of fish and customers, which is also best tested by speaking to someone in authority if possible, where a staff member - who may not be representative at all - fails to respond in an appropriate manner.
(Obviously, previous commenters know all this better than I do, but new people might read this, so I figured a little support for the 'speak up' contingent might be in order.
That and an alert for potential problems which might have been avoided if anyone noticing any sign of illness had mentioned this to the right person in the store.)