To preface- I will mainly be speaking to the stores freshwater selection.
First, the good: This is the best freshwater fish store in the Durham area. In terms of fish, the store carries the usual suspects (tetras, danios, angelfish, barbs, etc) as well as some harder to find species. The fish are happy and healthy and are generally of the live variety which is more than I can say for other chain fish stores in the area. Triangle Tropical also has a healthier and more varied assortment of freshwater plants than its competition. There smallish dry good section also has some tough to finds items and brands.
And now the bad: I do agree that the store is a bit dingy and sad. The lighting is very poor, the many tanks that are empty (mainly in the saltwater section) seem dirty and unkempt, the dry goods section isn't close to fully stocked and the staff don't offer a whole lot of assistance. There is some algae (which while unsightly is not an indicator of poor aquarium health) and are even a few little fishy skeletons lurking between some of the walls of tanks. While I know that chasing that overly-energetic fish that jumped out of the water and fell behind a 200lb 20g tank ain't fun, in this case it really must be done.
The reason I say that this store could be great is that remedying these issues would be fairly easy. A thorough cleaning, some new lights, new floor layout and a more proactive staff would make this place shine. They also need some more show tanks as this is what draws people in to the store and gets them excited about the hobby.
While I wait for my sage advice to be acted upon, this store will still be where I go for quality fish and plants.