I fear that you looked at no books or articles on starting an aquarium. You shop person was ineffective in telling you what should be done to care for your guppies. I would heartily recommend looking in the second section of Immediate Help. The link is in the upper right hand corner of this page. Especially look for cycling a tank. Google the Net if that is not sufficient

Try to find "The Everything Tropical Fish Book" Carlo Devito and Gregory Skokal.

Look in the big box book stores or try and order it through your library's inter-library loan service (you tax dollars already at work). On-line that and other great beginners' books can be found at placed like:
http://search.half.ebay.com/aquarium-fish_W0QQmZbooksQQpgZ19

Putting guppies in a liter container of unseasoned, uncycled water is nearly sentencing them to death. Every day they expel ammonia in their waste materials. That is very poisonous.

A quick fix until you can change the water is to get a water conditioner which both neutralized the chlorine and ammonia in tap water. You may have it in the bottle you think is a chlorine reducer. Please read the instructions and what it is supposed to do.

Go to a local grocery store and buy a couple gallons of "drinking water". Don't by RO, distilled or baby formula water because those have no mineral and will kill you fish immediately.

Mostly that water is RO water "rebuilt" with compounds of calcium, magnesium and (for flavor) potassium. That stuff will be ok "for a while" though obviously other elements and trace elements will be missing. As the water is used, refill from the cool tap and treat it with whatever what treatment your LFS (live fish store) felt was appropriate for your tap water. Leave the top off until the water is needed.

Right now you want to set a gallon of that stuff next to your "tank". When the temperatures have equalized, pour all of the water in the liter out, gently holding the guppies in. With equal gentleness add water from the gallon jug.

Usually we do weekly partial water changes, but in a container that small you may water to do partial changes almost every day. The day before the partial 25-45% water change (if you have the luxury of that time) try to leave the jugs where they will be as warm as the aquarium. In the case of a 10-gallon aquarium we would suggest adding the new water after gravel vacuuming the tank over part of the tank bottom.

The ammonia in the water is trashing the guppies' immune systems. That allows bacteria to run wild (in this case with fin rot). The same things would happen if you were to try a Betta in a vase without any preparation. Bettas are a little more flexible on small spaces, but keeping guppies or goldfish in liter bowls is like keeping a German shepherd in a closet: cruel, messy and ultimately fatal.

I hardly know where to start in making suggestions. If you know someone who has an aquarium, ask them if you could take enough water to refill your guppy container. Also, and more importantly, as them for some of the gravel from their tank. If they say yes, shake the dirt out of it (maybe scooping it with a fishnet) and put it in a clean, soap-less plastic bag. Keep it wet and add it to your guppy container. There will be some of the beneficial bacteria (which promote that nitrogen cycle you will have read about by then). That may help jump-start the cycling of fish wastes in that container.

You really need either to return the guppies to the shop or buy and equip a real aquarium, probably including a filter, heater, thermometer, gravel, light if you want even low-light plants, water conditioner, a net and test kits for ammonia, nitrites and nitrates.

Your "Log" submission here didn't get looked at by most of us until everything else had been looked at because we read left to right. (Midnight in my case and I was too weary and crabby to give you a n appropriate response. Like almost everyone with an emergency you naturally didn't read any of the instructions for posting or you would have offered a diary. That is ok in that just about everyone does that, but it costs in terms of response time.