SORRY about not getting back straight away .......... I have been having problems with my home computer and I had to get it sorted. Thanks very much for your replies and sympathies, I was (am still) gutted when it happened

here is (hopefully) everything you asked :
4'x2'x1' tank with lots of mopani wood, caves, (some) plants, small pea gravel.
UGF running the length of the tank with 2 power heads attached and a Fluval 3+ Internal filter.
It has been running for 2 years now with no problems...............
pH - 7.6
Nitrite - 0
Nitrate - 0
Ammonia - 0
These are all the test kits I have at the moment.... my LFS said that they would do.
5 Hoplo's, 6 Melatonia Lacustrus, 8 Cardinals ............... thats whats left now.
I have performed 4 (30%) water changes since it happened and (touch wood) I have lost nothing else. I hope the worst is behind me now - I don't want to go through that again.
DAMN I have JUST remembered I fed them garden worms the day before it happened - could that have been a possibility why they died.
Thanks very much - if I have missed anything please tell me.