Hi Platy77, It could be internal hexamitiasis (a parasite) Did you introduce any new fish lately? the disease is usually transmitted through their contaminated fecel matter, it can also be brought on by poor diet or stress (giving birth in livebearers is very stressful), most fish have these parasites already but they only have an ill affect when their immune system is weakened, poor water quality is also a cause, but as you mentioned your tank is well-maintained - what have you been feeding them up until now? Do you know how old they are?
Livebearers can be prone to these illnesses and it could be any number of parasites or bacteria, & if you feed tubiflex worms or bloodworms - more so with tubiflex, these can be full of bacteria/parasites. It sounds like you are doing the right thing & those meds should work - if you are using activated carbon it needs to be removed. I would keep a close eye on the fish to see if any outward signs of disease develop, like any sores etc. or they have trouble swimming. I would also continue feeding them the medicated food as well as other fish food when they get better.
You mentioned the temperature - how hot is it? often higher temps. help in dealing with these illnesses.
Are the fry in the same tank? if they are I would try & remove them if possible as fry often don't tolerate medications.
Sorry I wasn't any more help, I hope they get better!