If I could control my LFSs... they would have more selection of driftwood in all sizes, from small twigs to huge centerpieces. I've had to live with putting smaller pieces of driftwood together to form some of my aquascaping in my plant tank... it looked so ugly, that I purposely covered it in plants so that no one could see. It would be nice to have something "pre-made" by nature for people who suck at designing stuff

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A wide selection of food, everything from the cheapo stuff, to the expensive premium stuff.... dry to frozen, etc. A large range of sizes would be nice too... everything from a little bottle, to a big jug.
Every testing kit imaginable would be nice... not just for FW people, but SW as well. Once again, ranging from the cheapo stuff, to the premium stuff (its nice using cheapo stuff to monitor a cycle).
A wide selection of filters would be nice as well... and I mean wide. Everything from sponge filters, UGFs, power filters, canisters, FBFs, etc. etc. Many of the stores here only carry one or two brands of filters... rather than a wide range. The only store that carries a wide range here is Big Al's. I'd like to be able to compare filters side by side. Having them run on an empty tank side by side would also make it easier to compare the filters while they are running.
Bulk materials is good as well... like sponges which were mentioned before, poly fibre, gravel, rocks, etc. However, these items need to be kept low in terms of prices.... one of the points of having a LFS is so that your customers don't have to go everywhere to find stuff... quite honestly, if all I had to do was pay an extra 5 bucks for slate, I'd buy it from the LFS rather than go to a landscaping place... in the end, its cheaper for me, because I don't need to use more gas. For some reason, LFSs don't understand that charging 30 bucks for a little bag of gravel is insane... but thats just me (can't you just tell this ticks me off?

).
CO2 equipement would be nice as well. I know and heard of people who don't want to bother buying equipement from welding shops, etc. and would rather buy an all in one unit from a LFS... sadly, not many LFSs sell good CO2 equipement... the best I've seen from most stores is that commercialized DIY product Hagen makes.
Thats all I can come up with for now... however, do realize that even though these are the things we want, sometimes it just isn't possible to please everyone... the bottom line is, design your aisle so that it makes people happy, but at the same time, makes your boss happy (aka. makes money). Stocking some items just isn't feasible sometimes. You can't really help anyone if your aisle losses money, and your boss decides that you're incapable of handling your position.
HTH
-Richer