betta fish - ill

Question
We bought an apparently healthy betta fish yesterday from a respected shop.  
He seemed to settle in well - we introduced him gradually - putting in the
bag first, then mixing the waters, then letting him swim free.

Today the fish was lying on the bottom of the tank.  I checked the water
temperature (80 degrees).  He seemed to have a strange dark patch on his
side - just by his gill.  In the last few hours, this patch has expanded and he
has lost his blue colour from his head and from three-quarters of one side -
it is now a dark grey/black and it is encroaching on the other side.  Where the
black stops and the original blue starts, there is a thin while line.  

I think it is probably some sort of fungus.  The only medication we have in
the house is for white spot and the shops are closed here in the UK for the
next 12 hours. I don't want to treat him for white spot in case it does him
more harm.  Is there anything I can do for him overnight?  Should I isolate
him and should I  buy a preventative medication for the other fish in the tank
- 3 zebras, a guppy and a cory - if so, what?

Answer
Hi Sue,
 Check your water to make sure you have no ammonia or nitrates in it.  
 It could be anything from fungus, to bacterial to a virus.  
Treat your tank with Malafix, this is a natural medicine, and will prevent your other fish from getting this, if it is catching.  Some Bettas develop grey patches as they get older, but this happened too suddenly.  I would suspect the water first.  Put him in another tank with clean water, and use a good conditioner.  If he starts to look better, then you know that the problem would be your water...Bettas need very clean water.  I there is ammonia in the water they will react to this right away.
Hope this helps and that he does get better.
Lynda