by Ich! Yes the number 1 disease in fish tanks!
Ich is a parasite that lives in different stage and depending on the stage it's in the treatment will be different.
Methylene Blue will kill the 1st stage while it is attached to the fish and the free swimming stage but heat only will kill the encased stage.
Best treatment is Methylene Blue and increase water temperature to 86. There are some heat resistant strains that need the temp up to 90 to kill it. I find it easier to use the temp of 86 at first as most not all fish can handle this. Vacuum gravel everyday to remove any living spores from the gravel. I treat in this method for 7 days but leave temp elevated for 2 weeks.
Pic credited to Google
