Guppy - Lost Use Of Tail

There's every chance his fins are getting nipped... Have you observed this happening at all? Although I would be surprised if it's the neons who are doing the nipping... Maybe a competitive male Guppy?

Sometimes a fish can lose the ability to move their tail if they're severely bloated. However, in this case, it looks as though this is a (potential) case of finrot - some of my pygmy cories suffered from this last year in which they lost their tails and looked identical to this. In the case of the pygmy cories, this may have occurred during mating when they seemed to be rather aggressively amorous or by the two female x-ray tetras which I eventually discovered had a habit of following the pygmy cories when they were foraging in an effort to get any food morsels that they churned up. Some tetras do have a habit of fin-nipping fish with elaborate fins.

Whatever the cause in your case, it will be important to keep water quality scrupulously clean (ammonia and nitrites at 0, nitrates under 20). You could possibly medicate (especially if there's any sign of this condition getting worse / extending) with eSHa 2000 or Waterlife Myxazin but this would be better done in a quarantine tank with the affected fish in isolation. If you're unsure about whether to medicate or not and/or don't have a quarantine tank to move the affected fish to, then API Melafix is a milder option - it markets itself for treating finrot and for encouraging fins to grow back and for use in tanks when changes are being made, and I found it useful for that purpose, although I know others don't consider it more than an antiseptic.

Hope that helps.

I can't see the tail very well in the photo but the fins look clamped and bearing in mind whats just happened to the tank I'm wondering if maybe he's overeaten and has bloat and dropsy. Guppies are such greedy feeders.

I'd do what fcmf says and also consider feeding some blanched peas, they could probably all do with a good clear out. Although if he's reached the pine coning phase I'd be concerned for the little guy. Does he look oddly pigeon chested?