White patch on hillstream loach

Does anyone out there know what this mark could be?
It looks like the scales have lost their colour, the fish in question died, but was still in quarentine (3 days)he was fine and 4 hours later dead- maybe from fighting?
only other fish in the tank is another hilly and some dannios.

So sad been looking for these guys for so long!

 

Don't know if they can get it but Neon Tetra disease does that.

 

I'd think of Columnaris bacteria (saddlebackdisease). Quite more common than NTD and also known as "false NTD" having some of the same symptoms.

 

The other fish looks fine what preventitive treatment is there?

 

I'd say pristine water Gary. No treatment if other don't show signs. Is it a real hillstream-tank (cool water)?

 

There is a disease specific to hillstreams, they call it patch disease, not sure what it actually is but google it. You are a member of loaces online and they should be able to assist a little better I think.