The photos and esp videos told me a lot, thanks for those.
This is not an aquarium for a BGK, due to the brightness. These sensitive fish are nocturnal, never venturing out during daylight. They must have a dimly-lit tank with many hiding places. The behaviour of the fish in the videos illustrates its severe stress at being so "open." The light substrate gravel is a major factor in this, as well as the tank light, too few real hiding spots [sufficiently sized to accommodate the entire fish and then some], and floating plants for cover. I'm not saying this alone was the cause of the fish's demise, but it certainly would not have helped the fish adjust, so any other issues would only be even worse due to the stress from the environment.
BGK need soft, acidic water; a pH of 6.5 would be fine. You give your pH as 6.5 to 7.5, does it really vary this much? This would be another factor, as this can be highly stressful depending upon the time period. We can go into this more later.
BGK produce an electrical field around their body. I mention this because it limits suitable tankmates. Plus they are predatory, any small fish and shrimp will be eaten quickly. And they attain 20-24 inches, so a large tank is needed fairly early on since fish grow fairly rapidly according to the water conditions which are largely determined by space, and internal development can be severely affected in too small an environment.
The BGK may have been under heavy stress in the store, I think we can take that as a fact, and stress weakens the immune system. All of this probably contributed.