When you mention that you tested the water, what did you guys test it for? It is quite possible that there might be more ammonia/ nitrate or nitrite (or one of them) than is good for the fish. That filter in the water for the last week is great for knocking a lot of the chlorine from the tap out of the water and mixing a little oxygen in. Some nitrogen and ammonia in the tap water may also have been driven off or used to start the growth of "good guy" bacteria in the filter, on the gravel, tank sides and other surfaces. Filters don't really get going until a few grams of ammonium chloride or a couple of hardy fish are added to the tank.

Another trick of the trade is to get a bunch of gravel from another aquarist's healthy tank. If scooped up from their tank and kept wet (and above freezing this time of the year), it can "seed" the gravel in your tank.

I recently poured 15 lbs. of extra gravel in a tank with just this purpose in mind. Um.... you wouldn't live near Chicagoland, would you? ;)

A partial quick fix right now would be to add a water treatment that chemically bonds ammonia for a little while. Ammolock and the original Amquel immediately come to mind. Your LFS can probably do a better job of recommending something which will "lock up" extra ammonia for the time being. That slows down the development of the nitrogen cycle. It will still develop though and you fish will not be as stressed by an ammonia spike.

If you haven't been testing for any of those waste products mentioned above, call up you LFS and ask if they can test a water sample from your girl friend's tank. Also discretely inquire about any guarantee on the livestock. If they will replace dead fish in the first week, ask whether they want the bodies and if so, in the bag or "mounted" on a piece of paper towel or cardboard.

You probably have not had a chance to plow through the Quicklinks on tailrot, but please do so. An "over-the-phone" diagnosis like this is very tricky, but those should be some help.

Here is a link to a site with an example of tail rot - which can look different on different fishes.

http://guppyplace.tripod.com/Ailments.html

Here is a platy on the road to recovery from tailrot:
http://www.joshmadison.com/aquarium/images/fish/red_wag_tail_platy_2194.jpg

The following is a drawing of what I think of when tail rot comes to mind:
http://www.aquatics-warehouse.co.uk/extras/Info/Diseases_Cures.html

The references above will suggest an antibiotic you can use. A little salt (not table) and a water conditioner can be added. Don't mix actual medications though!

I fear if they have already discolored, your surviving guppies may already be on the way out. If that happens, keep the pleco (famous for their ability to defecate), give him an algae tablet at a time and let him run the tank though the nitrogen cycle. Continue to do weekly partial water changes (most of the bacteria are on solid items in the tank, not in the water). If everything looks and measures well, try again with a few guppies in February.

In addition to The Everything Tropical Fish Book (Paperback, 2000) by fish heads Carlo Devito and Gregory Skokal, I discovered Terry Fairfield's A Commonsense Guide to Fish Health (Barrons). At half the price of the first book, it may be easier to purchase. Terry's premise is that a correctly set up tank will do a lot to prevent illnesses. You sound like you have done a lot of things right, but his work may give you other useful suggestions.

Just for the record, here are some images of Hypostomus species. They are mostly of adult fishes and you may have to look a bit for youngsters.

http://images.google.com/images?hl=en&lr=&q=Hypostomus&btnG=Search

The following are Chinese algae eaters, so to speak. As a group, they are much more streamlined than those suckermouth catfish above.

http://images.google.com/images?hl=en&lr=&q=Chinese+algae+eater&btnG=Search

I feel mean spirited speculating on whether it is too late for your girl friend's guppies. It may be realistic.

In a general sense, what part of the world do you hail from? (IE, I'm from the suburbs to the south of Chicago, also known as Chi-ca-ga.) It may be that there is a guppy breeder or fish club near you. Often home raised guppies, from the neighborhood, are healthier and adjust better to your water. While that might be an end run on your pet shop, they may then be your source for fish food and supplies for the next several years and they could benefit from that too. :)

By the way, 76 degrees F / 24.4  isn't all that bad for your guppies. 77-78 is ok too, but if they are used to your 79-80 degrees don't change it. Consistency during hard times is important. (And cheap aquarium thermometers are often a tad off. But they do tell us if the temperature is consistent.)