I used to use hand warmers and luckily had no deaths using them but learned they’re much riskier to use because they have a much higher peak heat output and a shorter burn period, so you have a slight chance of overheating your fish at first and risk of freezing them later on if there are delays. They’re oxygen activated and it’s hard for oxygen to make its way into a sealed box so that probably slowed down the reaction, which is why I never lost using them, but this is still a much safer and mo