Some responses in this thread have been both laughable and sensible. Let me attempt to clean up a few myths, as well as offering my own advise on how to spot a good LFS
So, a shop has many tanks of fish labeled as "sorry, not for sale, under treatment". Does this make them a bad shop? Well, that depends really, how many tanks are a lot? If say 1/4 of the tanks are in quarantine, or all of these tanks are bunched close together, or are similar species, you can probably deduce that the retailers husbandry isn't up to scratch, as they are somehow cross-infecting tanks. On the other hand, if there is no pattens of infection, it's probable that it isn't husbandry at their end causing the disease outbreaks. Also, the stage of disease at which you can see in the tank makes a difference. If it's mild, it was caught early and the staff are on top of it. If it's all late stages of disease you are seeing, you can deduce they are avoiding taking stuff off sale until it's inexcusable. If there is no pattern and all the disease in in it's really stages, it's probably a good retailer. If there is a pattern of infection, all late stages of infection e.t.c., you can deduce systematic problems are behind the issues. Beware also retailers without tanks under treatment. Many retailers will remove and destroy fish that are showing symptoms over actually treating the issue...
Does the shop quarantine their fish when new in? If so, how long for? How soon after arrival do they start selling new stock? Does quarantine take place in the display systems? If they quarantine new stock, but it's for less than 4 weeks, they are just trying to look good. Quarantine is pointless unless it is carried out for at least 4 weeks, preferable 8 weeks, as the life cycles of most common issued mean that most issues won't show for 4 weeks... Also, there is no point in quarantining fish in a separate system to where they will eventually be sold, as the stress of moving is a main cause of disease outbreak. Fish also should not go directly on sale, they should be rested for at least 20 hours after arrival. I'd actually prefer to buy from a shop that only rests fish for the correct period, to buying from a retailer that messes up their quarantines, as fish are less likely to go over if removed from a stressful environment ASAP, or quarantined in that environment correctly
If all the tanks are connected together, does that mean that one tank with disease means they all have disease in them? In a word no, actually, when correctly set-up, a centralised system is less likely to spread disease about the shop's systems than individual tanks. When set-up incorrectly though, centralised systems are disease traps however... If the shop as a centralised system, you need to ask if it runs UV, if so, how much UV, and what kind of flow goes though the UV. Finally, you need to know the system size. You want to hear that all the system's water passes though the UV unit before it gets returned to tanks, and that all tanks drain back into the filter sump. You want a volume equal to 5X that of the whole system being passed though the system each hour, and you want at least 1W of UV for every 25-50gph of flow though the unit. If multiple tubes are used, you want them plumbed in in parallel. Also, check how old the tubes are, they need to be less than 6 months old to be effective. If the UV is being well applied, the shop is fine to buy from IMO, and is actually less likely to hit you with disease than a shop not running a centralised system. If the UV isn't correctly applied, it's time to start walking in the direction of a competing store...
Now, dead fish per tank. You can't get a set answer to this, as it depends on staff levels, tank decoration, busyness, shop size e.t.c. You will have to use your judgement here. You don't want to see a lot of bodies, but a few are excusable if they are busy, understaffed or the tanks are decorated. If you spot a body, make a mental note if where it is, and point it out. You want the staff member to remove it strait away, inspect all the other stock in that tank, and if there is disease, put them into quarantine and start treatment, otherwise, leave the rest be if they are healthy.
All the best
Rabbut