In theory, as the
Tetratonic is just a general tonic it may not have killed the bgk but I haven't used it or have a bottle to read the full instructions. Please read them as it may take the blame away from that. Personally I'd say it's due to small tank and multiple bgk's (more than one) as said above.
Until recently when a 6" bgk developed a weird lump/swelling i had 2 in a 4ft tank with no problems, probably as the 2nd one is 12" long and didn't see the small one as a threat but I DON'T advise it. This was only because the small one was a rescue one.
I've been keeping bgk's for about 3.5 years but I don't know everything about them but i'll help whenever I'm here if others haven't

Which is VERY seldom

Just stick to 1 in a 15g tank (if small) and try to use salts and heat as remedies for ailments most of the time but do as you have and ask here for help first. The bgk would prefer a bigger tank though

Elephant nose and BGK, well personally from what i've read it's best not to mix but as I've never tried it I can't say if it's right or wrong. They have different signals and can confuse each other when hunting for food. Obviously someone has them together and I pressume they ok but I'd say make sure the tank is large and do plenty of research first about the different sound waves

Hope the other bgk is ok