Is there any unusual algae growth? (brown stuff everywhere, cloudy water..?)
The fish you're purchasing from the LFS
might have issues before you get them home. Precursive problems like Ich and other parasites... there's no way of determining this without actually seeing the fish themselves (sometimes not even then).
If you're purchasing fish from the same LFS
- try a different store, or if they have each tank on separate filtration units -- try a different tank.
Regardless -- I wouldn't suggest adding anything to the tank until *after* it has been running for at least a month.