Since you have fish in the tank already, pretty much your only option is to change some water daily if possible- every two days at least.
Approximately 25%-50% of the water.
Treat the replacement water and try to get the temperature as close as possible to the tank.
Just a warning on the guppies, your few now will likely be 15 shortly.
(I started with 4 in a 10 gallon... now, including babies, I have 40+)
Live plants will also help your tank... Since you're in a bind I could send you some if you wanted. (I have a few that grow really fast and are really easy to take care of)
Although most people will disagree with me, I would try to get a couple snails and add them whenever you can. (don't pay for them, but try to get some free ones.)
If you begin to see algae (green slime on everything) don't scrape it away. It's kind of unattractive, but it will help use up the ammonia, just like plants would.
If you have an incandescent hood (screw in bulb) then you could just buy a small compact flourescent bulb in cool white, and order some plants from SweetAquatics.com (they're super cheap, and super nice)
The plants will absorb ammonia, produce oxygen, and make your tank beautiful. :)
I would probably reccomend-
echinodorus tennellus (dwarf chain sword is small and usually used in big tanks, but is also the perfect size for a background plant in a 5G)
those are all super easy for me... and grow like crazy.
All except the echinodorus can just be dropped in the tank and allowed to float. (although they'll look better if you stick them in the gravel)