Hole on the head?

Question
Hi Chris

Some months ago I lost an Angelfish to what I believe was hole in the head.  
Mainly because, well, it developed a hideous hole in it's face near it's mouth.
You could see right down it's gullet.

The man at the fish shop said there was no cure -- is this correct in your
experience?

Do you know what causes hole in the head disease?

For almost a year the tank has been up and running.
I have a 26 gallon tank (96 litres) and change 25-35% water every fortnight.
It's an AquaOne tank.  The filter is in the lid part with sponge pads and
ceramic beans-things.

Ammonia: Sorry -- I don't know.  Where did I put the kit?!
Nitrate: 0
Nitrite: I don't have a test kit for that
PH: 6.6

Since the angelfish died I have a peaceful tank with some neon tetras (10),
corydoras(2), black skirt tetras (2), beunos aires tetras(5), an otto and two
kribensis.  

Unfortunately one of the kribensis passed away this morning with what also
looked like hole in the head.  It's face near it's lip and above it's lips became
eroded.  I noticed a couple of weeks ago it's mouth seemed too open, like it
had a pebble stuck inside, but it didn't.

Is this a bug in the tank, or am I doing something wrong?

Many thanks for any help you can provide,

Regards,
Gillian  : )
Australia

Answer
Hi Gillian;

Wow, those sound like very extreme cases. Poor guys. I'm so sorry you had to go through that.

Hole-In-The_Head is a very difficult problem to treat. It is most common in cichlids. There are several suspected causes but the two main ones are diet and water quality. Another possible one is stray electrical current. Here is a very good web page about all these causes with treatment options;

http://www.duboisi.com/diseases/hith.htm

I hope everyone else is okay.....

At Your Service;
Chris Robbins