Got 3 Ghost Glass Catfish (Kryptopterus minor) on Monday, got up this morning and one of them had an opaque colour to the tip of its tail. Thought to myself better look into it when I get back from work as I was rushing. Got home about half an hour ago, checked my fish.... aaargh! The glass cat was dead. It had turned an off white colour, and lieing next to it was my Bulldog Plec... also dead, and very pale.

What it is? What's wrong?
The other fish seem and look fine, but I am concerned because I also got some Kuhli Loach, and 2 of them were next to the dead fish, but swam off when I got there.
Had a look at various tropical diseases but I am not sure what it could be. I thought maybe dropsy, but the scales were not raised, the eyes did not look to be bulging.
My water parameters are as follows:
PH 6.8, 25 degrees celcius, 0 Ammonia, 0 Nitrite, 0 Nitrate, 4 degrees GH. Did a water change on Saturday.
Can anyone help me? Is it contagious? How can I treat it? I am new to this hobby and any advice would be much appreciated.

Do you think it was already ill at shop? Should I complain?

Please help....
Regards,
Chris