Hello,
a silly question - what's the meaning of BGK?
If all bichirs are dying it seems to be the water quality!
No-one will accuse you anything ... in some cases it's the tap water that is contaminated!!!
I've seen you are from London - a mega-city. All mega-cities have tap water problems from time to time. Please check your water quality AND google about changes of the common water quality in London ...
An example: I think you know Cologne. The tap water in Cologne is so bad that aquaristics are nearly impossible!!! They are working on ...
But THIS is only one possibility ... any conspicuousness of the bichirs before dying?
Greetings & good luck
Uwe