I always point out dead, clearly ill, or slightly dodgy looking fish. These places can get busy, I don't know how often the employees get to stand and watch the fish. If the employees don't want to hear it then all they need to do is say "Thank you sir, I'll take care of it" and go ***** to their work mates when I leave!
I like a fish shop employee who quizes me over what tank I have, what lives in it, asks me questions, and gives me good advice. It makes me think they are interested in their job and their fish, and I'll go back.
My other word of advice for a fish shop employee (in fact any employee, in any industry sector), would be that if you don't know, don't pretend you DO know, you'll get caught out, or might harm fish. Don't say "I haven't got a clue", as it undermines your credibility. Try saying "Let me just check on that/make sure/confirm the information I have" and then go ask a more experienced member of staff. That's a question of professionalism, and if there is one issue I find with employees of fish shops, it would be this- purely down to the fact that the employees are often so young, and don't have the experience. Very many times the employees (and often the younger ones who are totally dedicated to the hobby) know their stuff but don't necessarily always present themselves in the best way.
A few ideas for fish shops to take or leave (and it's a bit OT, hope that's OK):
If you've got loads of kids running round banging on tanks on a busy Saturday, getting under your feed shouting about Nemo etc, why don't you try this: Agree with your colleagues that one of you will gather the kids together, and give them a bit of a tour of the tanks. Point out some of the prettier fish. Show them the ones that are easy to look after, or stay small. Warn them about ones which get very large, or are aggressive, or require shoals- anything that relates to their care above and beyond whacking them in the tank. What you would find I'm sure would be: 1. The adults would listen and you'd get less people trying to buy inappropriately, 2. The kids will be fascinated and will follow you around keeping out of the way of the serious purchasers and your work mates, and annoying you less, 3. The parents will appreciate you for it, the whole experience will be less stressful for them, and you'll both sell more fish on the day and get more repeat business. Schedule it- maybe hourly at the busy times, maybe once a day in the week. Use it as both a selling point and a mechanism for bringing order to your shop.
Try to increase on the customer service. Do you have reference books on hand for staff and customers? Or a PC restricted to some fish sites in the favourates? Most inappropriate purchases will be down to lack of knowledge of one or both parties. You can't be in 6 places at once, let the ones who want to, take care of themselves.
Why don't you have forms for the person to fill in asking things like tank size, current stock, planting, water readings, substrate type, filtration, desired fish etc. This gives you the opportunity to assess (a) that what they want to buy is suitable (and alternatives if not), and (B) selling areas (more plants, test kits, alternative/supplemental food, larger tank). Make it high enough quality and they can keep it and bring it back each time- kind of like a file. You get them doing that and yours will be the only shop people ever go to.
It's a worry that any fish shop employee has a problem with staff beyond people insisting on buying inappropriate fish (and in fact have you spoken to your boss about whether you should be selling like this). It is your job to manage your customers, screaming kids and know-it-alls included. You do have the opportunity to help yourself though
), but a lot of this sounds familiar... When you go to pets at home 2 or 3 times a week, you hear an awful lot of the 'Nemo!', and TBH I'm noticing a Little Britain theme 
