It depends on how valuable those fish are to you, are you willing to sacrifice them if it does not work? Do you do nothing and replace the lot? Tough issues we can not make right for you, we can just share info....
Here is what I have learned, though I admit I am new to guppies, but been keeping fish for years..
When fish poop is stringy white/pale, then probably have some internal parasite. Same with sunken gut, humpy backs, general lack to thrive.... Yup.... I offer what I did...this could work for others.... I was not having any real problems, some tetras just looked old, and skinny, ate fine- but decided to treat ALL my populated tanks because I was planning to add new fish to quarantine tanks... and want the best for all, some tanks are "breeding for profit" (yeah right, another thread, there....) ANYWAY!
All treated with API General Cure .... complete course and water changes, existing fish were no longer just living, I see them starting to thrive.
New fish are quarantined (4 tanks) for 30 days. All Treated with API General Cure. All guppies, except one tank has some baby plecos. However, one tank started to clamp and shimmy the day after last water change from General Cure, followed up with API Fuval2, ran the course of treatment and now they seem fine. Every one is about 11 days in, treatments complete ALL healthy, two tanks have babies (cant move them, we are in quarantine), but all is good!
I feed a wide variety of foods, fresh frozen, various variety of flake, live, and just got Doug Grays hi protein blend he sells on AquaBid, in the mail today (excited)! I feed 3 or 4 + meals a day, some times more, but tiny servings, so that also helps survival of the babies, signals the fish to breed , since food is quite plentiful, and avoids waste.
I change about 25-50% for water each week in most tanks. I have a 40gal pre conditioning tank, where I treat there, and then move it to the tanks so less shock to fish, having removed chlorine and heavy metals (API Tap water conditioner)
I hope this helps in some way
(corny video - but gives you something to laugh at!!!)