is there anything suspicious looking in the perlite at the bottom of the dead ones? perlite at bottom this isn't out of the ordinary per se, it's just a point of interest. also what soil brand?
fusarium
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you mentioned brown spots i think didn't you? dude you have fusarium contamination in your soil, or, im thinking the perlite at the bottom gave fusarium a breeding ground and they just took over the plants root system.