oooooh, this thread is hilarious! but sad at the same time.
can't say i have any especially stoopid stories to tell, but i don't frequent stores that have idiot customers that often (my fave stores are two small mom and pop shops with regulars that actually know their stuff when it comes to fishkeeping, thank goodness!)
one thing that does come to mind is not a customer, but an employee of a big box store. i went in and noticed that almost all the cichlids had extreme fin rot. i asked why that was. employee told me it was due to running out of and not having a certain additive (part of their corporate controlled dechlorinator/conditioner cocktail) in the tank. i can't recall the name of the chemical he said, but he stated that without this one 'ingredient', the fish get fin rot. in the same breath he said that all they used in their water was one certain chemical to remove the chlorine (i also cannot remember the name of that chemical he said). he also said the cichlids had been at the store a while and because they had been without the certain ingredient in the water the longest, that is why they had the rot and not the other fish (and yes, central filtration system).
i just said 'oh', and walked away, shaking my head. was tempted to grab a bottle of Prime off the shelf and make him put it in the system, as one 2 litre jug would've probably done the whole system, but i do tend to go there for dry goods once in a while......
