The best thing you can do, for yourself as a conscientious aquarist sales specialist, and for your employer - Petco's reputation, as well as to keep your customers happy & coming back (besides doing your research/ homework and being well-informed on the many facets of fish-keeping) is to spend every free moment you have during your shifts, ensuring, as best you can, that all the the livestock tanks in your store are kept as clean-looking, and free of debris, and any sick, dying or dead fish as you possibly can.
Check all the tanks frequently, immediately remove any dead fish that you see, treat any obviously sick fish right away (for say, ich, as an example), and place a quarantine tag on the tank, indicating the fish therein are under medication and not for sale.
Ensure you & everyone on staff does their job in terms of vacuuming detritus from tanks, cleaning up any algae, making sure proper water changes are being done, or if a central filtration system is in use, that it is operating properly & doing it's job.
Do everything you can, vis-avis your bosses and your fellow employees, to promote the maintenance of clean, good-looking tanks and healthy fish, without overcrowding them, so that they will present a very pleasant sight to potential buyers.
Hope you're following what I'm hoping to get across.
Oh, there are so many other things, but I'll stop here - I've already gone on too long.