hole in the head (x-posted to alt aquaria oscars)

Hi all,

I just purchased a used aquarium today. It is a 40 gallon and it has an
oscar, a green terror, a convict cichlid, and a good sized pleco in it. The
people that I bought it from told me that they only change the water every 3
to 4 months in it. I have never had cichlids before, but that does not seem
right to me. They also said that the oscar is developing what looks to be
hole in the head disease. From the pictures I've seen, I would also say
that is what it is. They had the fish for 3 years, and they were fine.
They also only feed one teaspoon of cichlid pellets every other day for the
whole tank. These are big fish, and that doesn't seem like enough to me. I
want to know how do I treat the hole in the head, should I feed them more,
and also how often and how much water should I change. They have had quite
a traumatic day today with the move and getting set up again (they still
have the same water they have had for 3 months, we just moved their water
also). Should I change it immediately or let them settle for a while?
Sorry for all the questions, but these fish are gorgeous and I want them to
be healthy and happy. Any help would be greatly appreciated.

Sarah