Sounds like maybe
Saprolegnia fungus.
I while back I had 5 RES in a 75 gallon aquarium that I fed baitshop minnows to every day, they would eat 15-25 small FHM a day. I would get minnows from different bait shops 2 or 3 times a week and never had any problems with my RES, I did this for six or seven months straight before turning the RES loose in my pond.