A lot of people have reported problems with this, so I'm reporting my findings. I don't care why you're buying neons from a petstore, or the relative merits of petstores vs. other sources, only that you and your fish have a good experience.
The first issue you must avoid is shock.
Neon tetras are very sensitive to changes in water parameters. In particular, salinity. A long time ago I noticed that all neon tetras from one store tended to die, while the ones from another store tended to survive. Later, it reversed completely.
I eventually figured out the ones that tended to die were from whatever store was using aquarium salt at the time, and that transferring them to my non-salted tank is what was causing death. Switching to a nice, long drip acclimation solved this, and allowed me to have success with neons any store.
I recommend using drip acclimations with neon tetras, regardless of source. Don't skip it, even if you believe your source isn't using salt. I've had petstore employees tell me they're not using salt, even when a cup of salt is plainly visible in the tank. If the cup was recently removed, you'd never know the employee is giving you false info. It's possible some sources also might not want to admit they're using salt, as that might suggest to a buyer that there is a reason for it - like a known disease, rather than prophylaxis.
As a side note, otocinclus are just as sensitive to changes in salinity. Use drip acclimations for them as well.
The second issue you must avoid is bacterial infection.
The drip acclimation was, for a long time, all that was necessary. But in the last year or two, things have changed. A lot.
In fact I went through five different attempts to get new neon tetras from various stores. Within a few days, all developed progressively spreading loss of color, as if their outer layer of skin was peeling away. In the end, not a single one survived. And going by posts I've seen here, this is now a common occurrence.
I suppose I could have just stopped buying neons from petstores. Maybe that's wise, but I like a challenge. And after the second failed attempt, I decided I was going to figure this out, no matter what it took.
Judging by the speed at which the disease progressed, I concluded it must be bacterial, probably something latent brought on by the stress of shipping and repeated tank changes. Perhaps flexibacter or columnaris, but I'm not sure.
Maracyn (Erythromycin) and Maracyn II (Minocycline) did not cure or even slow the disease, either alone or in combination. Neither did salt (introduced slowly), methylene green, or Kanamycin medicated food.
What DID work is treating the quarantine tank with Kanamycin and Nitrofurazone, in combination. Do not wait until you see symptoms, start as soon as you get the fish. Use the recommended dosage for five days, with a 33% water change before each dose. For my powdered medications (from www.nationalfishpharm.com
), I used 1/8 teaspoon of each per five gallons. Perform any other QT procedures you consider standard afterwards.
After discovering this combo, I've acquired three batches of neons, with 100% survival - where before I was consistently seeing 0%.
Some other notes.
I've since also added Metronidazole to the combo, and use it any time I suspect or anticipate a bacterial problem. This more aggressive combo both has a broader spectrum, and reduces the possibility bacteria will develop a resistance. But this last addition is NOT required to fix this particular problem with the neons.
I don't know whether this combo knocks out the biofilter or not. At no time did I detect any ammonia or nitrite. But all my tanks, QT included, are packed with enough rapidly growing plants that a working biofilter is optional. If you try this in any tank where the ability of plants to reduce wastes on their own is in question, please perform your own tests, and report your results here so that others may know what to expect.
Nitrofurazone is very poorly soluble in water. There's no way to adequately pre-dissolve it in a small amount of water like in a cup, as it takes at least 4.2 liters of water to dissolve 1 gram of nitrofurazone. If you keep your replacement water in a bucket, you could dissolve it in there. But if you add it directly to your tank, it will float and get blown around, slowly dissolving over a half hour or so. This made me nervous, so I watched closely. A few neons tried to eat it, then immediately spit it back out and did not try it again. In the end there were no noticeable negative effects.
I read a warning that Nitrofurazone should not be used in a planted tank. Tried to look up why, and found reference that it may block photosynthesis. Also found one specific report of it melting vals. I do not have any vals, so I can't verify that. It had no effect on 20 other variety of plants. I do have some Echinodorus Angustifolia (Vesuvius Sword) that is currently melting, which you could mistake for corkscrew val if you weren't familiar with it. But this didn't occur until many weeks after a treatment, so I'm not sure whether the Nitrofurazone is responsible, or if it's in response to some other major changes in that tank.
I've tried cardinal tetras in the recent past, apparently losing them to the same disease as befalls neons. While I haven't tried cardinals again since discovering this antibiotic combo, I think it would probably improve their survival rate too.
Hope this is helpful.