If you have been running your aquarium in the neighborhood of 80 degrees F/27 C those are not young guppies. It is possible that they are just dying pretty much of old age.

If you want to feel a little guilty, it is possible that their problems were triggered by a little lapse in keeping up with your regular schedule of partial water changes. Summer temperatures can also set back a fish's immune system too. (Heck it happens to wild fish in the US in the hot months too.) Since partial water changes are the most difficult part of the hobby for me, I don't have much room to be very critical. ;)

I do think that time is about up for them. Continue to keep them separate from your other fish so that they are not contagious and let them live out what is left of their lives. It may be a horrifying though, but euthanasia to put them out of their misery is an option too. I don't like to think of pet fish as disposable (like an old or unwanted toy). But sometimes the fish is so clearly suffering that it may be time to end the pain. If you would rather let them continue until they die, that is probably what most correspondents here would do most of the time. (Me too most of the time.

There is a bit on euthanasia in Immediate Help.

If you want to try helping them, sometimes a salt bath will remove fungus. Most fungus treatments seem to treat for both real fungus and colunaris. I have found the latter really tough to deal with once it gets going. Spit tails may be age or less that ideal water. Keep up the water changes. Even adding a fragment of a vitamin B tablet to rigorously changed water has been recommended.

You sound like a fairly experienced aquarist. Tell us about yourself or at least what your decide to do about the females.

By the way, by consensus, we effectively dropped doing logs on Guppylog. Maybe the software writers are disappointed, but if everyone submits diaries, they go to the front page immediately and you may get a quicker response.

The other day I saw your name in the Who's Online? box and noted that you spent a little time there. I was surprised that you hadn't left a diary. The log of course was what you were working on. :)

Here's a report on Stan Shubal's experiment with fancy guppy life expectancy and temperature. In the wild, it is a rare guppy out of a batch that even lives six months without getting eaten.