One of the e-mail lists I subscribe to has had a hot discussion lately on preparing water for the aquarium. It can get pretty involved, especially for those of us who would never be confused with a chemist. ;)

Miskairal is also "right on" in asking for more data. You might refer to the (actually 40 questions) in the Immediate Help article and response in:
20 Questions for new fish owners or owners of sick fish By angelhologram

Among them, what is your general pattern of partial water changes? [Mine is terrible, lest I sound too judgmental. Too much time on-line telling others to change water. ;) ]  Is your water “seasoned” at all? Has your aquarium completely cycled over the last 6-9 weeks?

Having gone to several web sites and checked one of my more reliable fish disease books, I really don’t have a (single) clue what has happened. Burgess, Bailey and Excell’s book notes many of the possibilities, of what is happening to your fish. These might include bacterial infections, perhaps as skin ulcers; the result of Haemorrhages (their spelling) again perhaps caused by bacterial infections; tumors of some sort; viral infections (against which there are no defenses except to try and keep their water and environment as good as you can) or damage caused by ectoparasites. (One could ask if you have seen any Argilus (fish louse), Lernia (anchor worm), Leeches or flukes around the tank – fairly visible critters which may have been hanging on to the side of the fish. Would I know what they look like swimming around the tank? Probably not either!

It could also be a very bad case of Hexamita or Hexamitiasis (similar to the hole-in-the-head disease of cichlids), which is again provoked by dirty water. It could have been an injury while the fish was transported or inflicted by a tankmate. A really nasty thought of mine, it could have been caused by an internal parasite EXITING your guppy. Some come to mind, but that is so speculative, I’d rather not even hazard guesses as to what they might be. Perhaps some sort of alien. ;)

So TLC is probably your best chance in helping your fish to help itself. If you have a quarantine tank where the water will still be warm and at least as clean as where it is at, you might try that. If you are afraid of that malady spreading to your other fish, you may wish to read and consider the Euthanasia article in Immediate Help. I want to hasten to suggest that fish should not be considered “disposable pets!” No animal should!

I would try to do what I could for that fish, but may have the advantage of more space. TLC, mostly in the form of doing every thing you can for improving and maintaining water quality and feeding well without over feeding, are especially what you can do for that animal. Antibiotics can prevent the spread of sores or heal some wounds. However many of them also trash the nitrogen cycle in the aquarium, yet further stressing the fish. :(

I do think you guppy may have a chance if you are able to work with the water and if tankmates leave that him alone. If there is a place (even a plastic plant or clean, soap-less, unraveled plastic pot scrubber) it can shelter in, that may also be beneficial.

There is a sort of human analogy from the American Revolutionary war. A soldier was shot in the stomach. Upon investigation, it was a “clean” wound. Not only did he make it (in an era before antibiotics and even washing hands), but a curious doctor kept shoving things like meat (on a stick or string) into that hole and began the process of investigating how stomachs digest food!

Good luck and all the best, Jacki!