Plants: everything from low to high lighting.
Different breeds of Tetra.
definitely all kinds of algae eaters. You might even want to have your own section for that: flag fish, SAEs, amano shrimp, the works
Display tanks of all sorts: planted, reef, monster tank, vivariums, ponds, specific biotopes (South american, african, asian, etc)
correct labeling of fish when tanks are mixed
staff that knows a lot, but don't act like they know everything. (i found this great place, but has horrible staff, so i seldom go there)
cheap alternatives
Are you trying to open a fish store?