Thanks for all your help guys. I had to go back to work and only now getting home 9 hours later. All the rest of the fish are accounted for and seem fine under the moonlights.
Unfortunately I don't have a Q tank as I've nowhere to put one at the moment although I'm trying to work out where I could fit one.
I won't introduce any new fish for a while and see what I can do re treating the tank.
This is probably one of the most frustrating hobbies I've undertaken, even after golf!, but theres always something to keep your spirits up and the need for that perfect tank.
Thanks again all for your help.