blisters on fish

Question
I have 4 large oscars that came with the 240 gallon tank that I bought.  They have been really healthy but this morning my son noticed what appears to be blisters starting on the top of two of the fishes heads.  They are not tall blisters but they are dime size a look like a blister.  One of the fish has more than the other.  The are a mated pair so do not know if that is why they have them and the other mated pair does not.  I know my husband did about a 4-5 inch water change the other day as our water was turning yellow.  He treated the water before he put it back in and he used luke warm water from the kitchen.  That is really the only change there has been.  This paticular set spawned about a week ago but they ate the fry.  Do not know if that makes a difference.  There is four oscars and 1 arawana in the 240 gallon tank.  No live plants and I have two bubble wands running all the time.  Please help!

Answer
It could be Lymphocystis which has no treatment but has been known to go into remission.
Or it could be Glugea or Henneguya. This has no cure and it is best to destroy the fish.
If it's cottony it could be a fungus which is treatable with fungus medicine.