I would get a water test kit yourself, then you know what the water parameters are. You will also understand the nitrogen cycle much better, and understand the chemistry behind what keeps your tank healthy.
With the word of your lfs that your water is fine, i would suggest that there is perhaps some unseen illness in your tank, especially if there are no external signs of injury. Another reason could be that the fish that are dying are being bullied by other members of the tank? Gourami's are particularly prone to bullying, I lost a honey gourami last week, and i'm almost certain it's because it just hadn't been feeding normally, as the surviving one of the pair had been chasing it into a corner almost constantly, and had grown to double it's size! I also had a dwarf gourami that harassed a platy for much of their time together in the tank. These fish can sure hold a grudge! Sit down and watch your tank for a while, see if there are any obvious bully's in the tank.