Chasing each other within their species is normal for glofish.
The ammonia level is at 0 ppm? I really appreciate your measuring that. It makes it a lot easier to diagnose your fish. Because the ammonia is at 0 ppm, the odds of the fish dying of ammonia poisoning or of a ubiquitous pathogen that invaded because of their chronically weakened immune system is much decreased.
What did the bodies look like after death? Did they float or sink? Were their eyes clear or clouded? Were the abdomens intact or were they blown out? Were there any red spots? Fuzzy spots? White flecks? Raised scales? Slimes? Did they itch themselves on tank decorations before they died? Were they eating regularly up until they died?
Rapidly decomposing bodies are usually the sign of a pathogen. Abdomens blown out can indicate an internal pathogen, while red or fuzzy or white patches can indicate an external pathogen. Raised scales indicates internal organ failure before death (dropsy). Slimy coatings on the bodies can be caused by massive bacterial colonies sometimes. If the fish itched themselves on tank decorations before they died, they likely were bothered by an external parasite.
Here's what you do: Wait. Don't add any more glofish to the tank. Wait to see if whatever is killing them is affecting your other fish species, too. Don't add any medications; without knowing what to treat, adding a medicine will most probably only sicken the fish with its side effects without damaging the pathogen. Keep monitoring the ammonia and make sure it stays at 0 ppm.
It's very rare for a disease to kill without symptoms. Keep an eye on the remaining fish. I'm going to link you to a diagnostic website. And remember: Only add medicine after you positively identify the disease. Adding medicine at random, and especially mixing medications, will most likely stress the fish.
Here are the diagnostic dichotomous keys. Use the tabs at the top, for example Bacterial Disorders, Behavioral Disorders, Body Cavity, Changes In Color, etc. Link: http://www.fishyfarmacy.com/fish_diseases/eyes.html
If your fish don't match any of those diseases (and that is basically a complete list), then you might have to prepare yourself for the rest of your glofish being slowly wiped out. It's awful, but it happens. The main thing to keep in mind is, Don't buy any more glofish. Buy a different, completely non-danio species, one that is not related to glofish at all. If those are the only fish getting sick and dying in your tank, why replace them? That fish species doesn't like your tank conditions. Instead, pick a different fish. Buy your new fish from a different store, from an entirely different seller. It's possible that the fish you had that were dying contracted a pathogen specific to their species before they ever came to you. It's happened before, with neon tetra disease. Instead of risking letting whatever those glofish have get transmitted to new danios, get a different species altogether. You say that your other fish are fine, so, stay away from danios for a while. Maybe once whatever it is loses its host for a while, it'll die off, and your tank will be safe for glofish again in the future.