There are a few possibilities...

1) Your first fish could have been being nipped on once he died by the other 2 fish... which would cause the red spots. Otherwise, I'm not positive what happened there.

2) It is possible that your fish are dying because the pet/fish store houses their puffers in freshwater, and then you are putting them in brackish water without acclimating them.

3) The lighter patch of skin could have been a fungal infection? Not sure.

It is possible that the worm had bacteria that affected the one fish, but unless the fish was already susceptible to infections (stress or already sick) it should have been fine. Since you mentioned that the little one was being picked on, I'm assuming that's the one that died... could have been a combination of the stress from being picked on and a fungal infection/bacteria infection be that from the pet store or from the worm or just general bacteria in the tank.

I don't think your fish are dying of nicotine poisoning, but I'm sure small amounts are being absorbed into your tank.

How much are you changing when you change your water? If you are changing more than 20% every 2 weeks (or 10% every week) it could be an issue with the beneficial bacteria.

Hope this helps.

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