Chances are you will need to increae your weekly partial water changes (of treated, maybe seasoned water. May I suggest that if you have been doing those water changes, that you do a 30% water change in the next day or two, then a 35% parttial change in a day or two after that. Up the percentage to 40% and even 45% if you can.

Medication may help the fish and then kill them as the nitrogen cycle is wiped out by any antibiotic. The first step could be cleaning up the water so that the guppies' immune systems are strengthened. If the water is dirty (ammonia is clear) and remains dirty, your guppies will continue to be plagued by bacterial infections. Bacteria usually can only attack guppies when the water is not all it can be, That causes their immune systems to be crippled and the bacteria, which usally can be held at bay, can then successfuuly attack them.

You might also look into a treatment bowl (if it could be placed upon the tank (to keep it warm) or even check out the idea of a medication/quarantine tank. I feel bad mentioning those because they do involve a cost in terms of money, space and effort.