I would be wary of what a LFS is telling you, however some really know their stuff. Especially small, independently run shops that specialise in fish.
What I would possibly recommend is you tell us your location (just city or something, not house number and road

), then people may be able to suggest reputable fish shops that should guide you well.
With regards to meds to treat your fish, without knowing the exact cause of death it would be a stab in the dark. While it may have been a stomach infection that killed your other fish, without more information ie picture, I would stick to the regular water changes and concentrate on keeping the water as clean as possible.
If you truely suspect a bacterial infection (google some common fish illness/diseases and try and match it first), it might be worth treating with a broad spectrum medication. I'm no expert on disease diagnosis or treatment, perhaps post in the 'medical emergency' section of this forum and ask for a wide-ranging anti-bacterial treatment - Melafix springs to mind.