Sound's to me like the associate has the customer's right where he want's them either out of ignorance,or planned.
Fish are dying,,sell em more fish and bacterial supplement' s that may not be of any real benefit.
More fish die,,,sell em more fish,and possibly some medication's to then dump in the tank.
If the tank's where fish are dying are newly established,, then removing the filter material can often result in a good portion of beneficial bacteria that is slowly developing being removed as well.
Add to this the likelihood of over stocked tank's,overfeeding,and fishes are under considerable discomfort from too much waste ,food,entering the tank without sufficient biological filtration to handle it.
In addition to this,,,perhap's fish are not suited for particular tank due to water parameter's falling outside their preferred range(pH,GH.KH) or temperment's.
No problem,, He can then sell them potion's,powder's that will quickly adjust the water's parameter's and if fishes die,,,sell em more fish.
Some folk's travel on this merry go round for as long as their interest,determination,and dollar's hold out. Other's begin to question why,and at this time they finally begin their research which hopefully lead's them to search out sites like this for information that they may be lacking.
Yet another group of folk's don't care,, they are more interested in pretty color's of fish and will eventually have a cycled tank out of attrition.(lot's of dead fishes).
I shop for food's and supplies at the chain store's and often hear very poor advice being handed out, but I bite my tongue for most often,,,Offering perhap's differing advice, is akin to suggesting that their,child,dog,is ugly.
Bout as much as you can do and not create tension at the workplace, is suggest that maybe the person's might like to visit a popular fish forum like this one where they may be able to gain knowledge that doesn't cost em anything.
Nearly all folk's like free stuff