Help identifying disease in an unhealthy baby guppy
Recently I had gotten a new guppy, he is fairly young and I have him housed with an older female guppy and two older mollies. Being of the same family I had not expected any aggression to occur as usually there isn't any between mollies and guppies. However an hour prior to receiving the guppy from a local pet store which treats their water very well, I had realized the guppy had popeye in one eye. After letting him accumulate to the water and put him in the tank he seemed fine and I treated him for 6 days with a medication called maracyn. Which recommends to treat for 5 days, however over the past week he has not seemed to heal and it has seemed to get worse as I have noticed a mask of white around his face, which if I suspected was a fungus , should have healed since Maracyn treats it. I am thinking of changing water today as I do weekly but I am concerned and do not know the further steps to take for the fish.
I'm not sure about the parameters of my tank, I am working on getting testing equipment soon. I've had the tank set up for a year now multiple inhabitants have lived and died in it and it has been cleaned countless times. He is housed with an older female guppy who is rougly a year old , and two mollies who are bigger but i have only gotten them recently. I am using an API superclean 10 internal filter. I usually clean my tank every week or so, I want to clean it today and i usually do a 50% water change . The fish had been accumulated to the water for 10 minutes before I unbagged him and put him in the tank. There has been no new decor added to the tank for a long time, just a skull decoration I have had for a long time. And the fish have been fed BettaMin, as I previously owned bettas. I am putting a picture of him on the thread.