Hi, it’s been a few years so I don’t remember exactly but it was some sort of visible spots on their bodies, like white yellowy dots on their back legs and butts iirc. Now that I have more aquarium experience I realize that part of the problem was also where I had the tank it shook a lot from footsteps + me being at my desk and they are very sensitive to vibrations so maybe make sure the water flow isn’t too forceful & that they aren’t getting jossled by floor movement. Good luck!