How many Zebra Danios have you got in there? Its quite common for the smaller danio species to get quite nasty with each other when in small groups of less than six, as I saw late last year with my Pearl Danios (I ended up buying another six to make a group of ten), they become almost territorial with each other and sometimes start nipping other tankmates too.
One of my Pearls ("Mr Stumpy") was found with pretty much no tail left at all, upside down behind the Fluval U4 filter in my 620T before Xmas, when I had a group of ten temporarily in there. I had very little faith he would survive the night, but I moved out all his "friends" so that he just had my little Filament Glassfish for company. With good food (including more treats of defrosted bloodworm than normal) and ~10% water changes every one or two days, Mr Stumpy regained most of the upper half of his tial fin within three weeks, at which point I reintroduced him to the group in the Rio240, where he made a complete recovery just before "mussels" collected them the other weekend to join his 6x2x2 tank (which now includes ~25 Pearl Danios).
Going on the above, I think your Zebra could recover, but this is more likely in a quarantine tank by itself.
I believe Guppies like sub-tropical temps (18-20C) like Zebra Danios, but without looking at a profile I suspect the Neons need ~24C. Keeping the Guppies and Zebras in water higher than 20C will drastically increase their metabolism, making them "greedy eaters" (so they excrete more waste) and hyperactive (they will die of old age before their time).
In addition, many of the small danio species like Zebras are very active fish for their size, they are far too active for a 10g tank! You could really do with buying at least a 3-foot [ideally 4=] tank for them (Clearseal range is good price at Pets At Home for new tank, or scour Ebay ads local to your location). They love current too, 6-8x the water volume per hour through a filter and/or powerhead.
