Everyone is saying poison, toxin....
how about the fact that the tank
Looks extremely immature, a new person was feeding the aquarium which almost guarantees over feeding. Or feeding too much food too quick, ammonia before he even left at .25.
I’m going to go out on a limb here and say the instability in the tank, lack Of Maturity and biological diversity couldn’t handle the heavy handed feeding, im sure it’s possible a closed up house while on vacation lowered PH, then maybe everyone panicked and opened some windows, raised ph in the presence of ammonia which made ammonia toxic.
Just some thoughts
those rocks look
Like they don’t have any live on them
That tank is unstable
lesson learned, go slow
I wouldn’t be so quick to blame a couple of shells