I work in a big box pet store too, it's what got me into the hobby actually, so this answer is coming from a fellow employee and also a customer to other fish stores.
Be available but don't bug them. I always greet my customers, ask if they need help, and tell them that I'll be around if they have any questions. Other than that, I leave them alone unless they look extremely confused. I expect the same from other pet stores and I honestly get kind of offended if a worker clearly notices me and doesn't at least say hi.
What someone else said about good labeling! This ties in to the first one. When someone new to fishkeeping is picking out what fish they like, I always show them four things on the label so they can make their own smart decisions easier. 1) What community the fish belongs in, 2) If the fish is schooling or not, 3) Minimum tank size for the fish, and 4) Adult size the fish gets. This way the person can learn as they go instead of cluelessly pointing to fish or asking you 1000 questions. We (Petsmart) also have care guides hanging on a rack near the fish and I tell them to take as many as they want.
Don't just push sales. To me this means suggesting better methods than adding a bottle of chemicals to their tank (heat for ich, changing light schedule for algae, etc), not selling them fish unsuited to their tanks...that one is a big one for me and I'm sure my managers aren't always happy. I always ask how big of a tank they have, what other fish they have, how long it's been running, etc, but not in an interrogating way. I just try to make friendly conversation and also figure out how knowledgeable they are, and if their tank is suited for whatever fish they want me to catch. Then, remember the customer's faces so you're not asking them every time lol! I also tell people often to wait on certain fish either if I have noticed a particular tank has been unhealthy lately, or if they want to add sensitive fish to an uncycled tank.
New employees should shadow learned ones for a lot longer than you'd think. I know, I know, payroll doesn't always allow it. And you might not be in control of this. But they seriously threw me out onto the floor with the most basic understanding of fish and in turn I had to give customers advice on. Thankfully I learned, but it sucks for everyone involved if the employee doesn't fully understand what they're preaching.
Effort!!! I went to a different store the other week and asked the employee to dig around in the gravel of ONE tank to see if any khuli loaches were hiding in there as I couldn't see any...he looked at me like I was the devil incarnate but lo and behold, there were 3 little munchkins burrowed in there and I got to take them all home. Don't be like him. Always be willing to give the customer what they want, even if you have to stick your whole arm in the water.
Like others have said, dirty tanks or dead fish. I completely understand that taking care of a pet store is a lot of work, and a lot of people don't realize how little time we get to clean ALL the animals, not just fish, as well as a ton of other tasks and THEN on top of it, making sure no customer goes unnoticed. Time management is a big thing. There will always be something that you're slacking on, but try not to let it get too bad. If I see a section of tanks with a ton of algae on the glass, I'll take 3 seconds to half-assedly wipe them down so they at least look presentable.
Isolate unhealthy fish. There will always be some. Put them somewhere else.
As far as stocking goes, I would just look around these boards to see what you hear a lot of buzz about. I think the top sales at my store are livebearers, neon tetras, and cory catfish. Oh, and "sucker fish." Everyone wants a fish to clean for them. Having a stock of SMALL plecos (bristlenose, rubberlip) and otocinclus will be good because you can steer them in that direction instead of grossly overstocking their tank with a common pleco.
On the topic of stock, it also turns me off when I go to a store and they have 100 tanks of different cichlids, and then 10 of tropical community fish. Having different communities is important so you can appeal to a larger amount of people! At Petsmart we have a goldfish section, african cichlid, south american cichlid, semi-aggressive, and tropical community.
I'm trying not to make this too long as I have a tendency to be wordy, so I'll stop now lol...PM me if you want anything else!
