Thank you all for your feedback and help, the FAQs I've been pointed towards
are very helpful.

Unfortunately as expected, all of the fish are dead bar one, who will be
joining the big bio-orb in the sky very soon by the looks of things.

Now, should I clean out the tank completely: the gravel, castle, and glass
before I start again? I don't want any new fish to catch the disease that
killed these guys, so I'm assuming I need to start the whole thing again and
let the tank mature for 3 weeks from scratch.

Ideas? The water quality is good, all the tests are fine, but I don't want
any nasty germs and bugs to still be attached to the gravel, plants and
glass...

thanks!

Robbie