Actually all the employees SHOULD have a reasonable understanding of most of the fish sold.
No, you can't know everything, but basic knowledge about the species stocked - such as general temperament/compatibility with fish it is commonly kept with , eventual adult size and what gallonage of tank is needed for that eventual size, and wether or not it should be kept in groups, pairs or singularly, is really a must if you're going to sell something to a variety of customers, around 70% of whom will have absolutely no idea what they are doing.
If the store has separate sections, say freshwater and marine, then hiring someone who knows about either one section or the other ( expecting someone to know both to a competent degree is probably pushing it ) is really rather needed. Even more so with marine as things seem to have a tendencey to go wrong a lot more quickly than with freshwater.
If a species comes in that hasn't been sold before or you don't know much about, READ UP ON IT.
Same with hardware. Basic knowledge of how a bog standard filter works, and why it is needed is essential if you're going to sell it to someone. Same goes for heaters, lights and other basic equipment. If a new product comes in, READ UP ON IT.
Get as much info as possible on what you're selling, make a handbook and make sure any new employees have read it thorughly.
Hiring people who at least have experience in keeping fish ( and enjoy it ) and using the basic equipment of the general type you're selling will benefit you, the customers and the fish far more than hiring some dimwitted spod who just wants beer money for the weekend and will tell a customer god only knows what in order to get a sale.
No one is expecting a world expert on every species known to science,with 12 masteres degrees in various branches of icthyology, but someone who has at least some experience of fishkeeping, and has read the correct information on what they are selling is hardly an unreasonable thing to ask for at all.