Dwarf Gouramis.

Dwarf gouramis are awesome fish. But they DO die easily. Not because they are hard to keep but because of some chronic virus (Dwarf gourami iridovirus) that is present in captive bred stock. The virus shows up as dark spots, then subsequently ulcers and stuff that I can't remember. My advice would be to pick them from tanks that do not display a single sick individual, that should be your best chance.