There is a difference between a glow fish, which is genetically altered to produce the effect and a glo-lite tetra which is a species, Hemigrammus erythrozonus.
Next, there are several things that would explain what happened to your fish. Dropsy is more of a description than a specific disease. However, there are several common causes of what you saw.
Constipation- This is one of the ore common causes and easiest to fix. the fish is simply not pooping because it is plugged up. if the fish is still eating then daphnia or a crushed pea can help. If its not eating some folks feel that epsom salt, not sodium chloride, it the best treatment. Epsom is basically magnesium. Not everybody agrees that Epson is helpful. but I have used it now and then with decent results. But it will make one's water harder and care needs to be taken to add it gradually as too large of a dose dose can do more harm than good.
Internal Parasites- These need to be treated with an anti-parasitic med. Parasites come in different kinds and different treatments can be needed. Note, here we are looking at internal parasites. This may help some, it is a link to the Merck Veterinary Manual's section on
Parasitic Diseases of Fish Scroll down to the sections on
Internal. A few of my other links have gone invalid or are down due to rewrites.
Bacterial Infections- These also will need treatment with meds- antibiotics. The constipation here is normally caused by an infection elsewhere causing swelling which presses on the digestive track which blocks it. These can be difficult to diagnose since the only symptom is the swelling of the fish which is often not enough on its own for an accurate diagnosis. Of course, the infection will sometimes be in the digestive system itself.
Genetic/Physical defects- There is pretty much no solution for this sort of thing, especially in such a tiny fish. But animals can have these sort of issues just like humans. The can also suffer heart attacks or strokes for example.
Where one treats a fish depends on the problem one is treating. Anything that is contagious or can spread requires a whole tank be medicated. Individual not contagious problems should be treated in an H tank. Anything one adds to the water alters it to some degree. This will usually cause some amount of stress in fish. More serious levels or prolongeed exposure results in a weakened immune systen and a lowered ability for a fish to ward of things it normally would.
Finally, for most of us in the hobby, dignosing fish ailments and treating properly is not simple nor easy. Ich is easy to spot and treaments are well known, but most issues are not this simple. I have been at this for 19 years and I am still pretty ignorant re diagnosis and treatment. I lack the education and equipment eeded to diagnose. Unfortunately, professional help is hard to find when it comes to treating a single tetra or anything else for that matter. Most vets do not work with ornamentals so you need a connection at a University which has a department which teaches this. Not so easy to find. And most of us lack the desire and/or time to get educated. A few years back an event (with a 40 year history) where I used to vend offered a seminar in fish disease diagnosis from a pro. They cancelled it because of lack of interest. I did sign up and was a bit disappointed.
