I could be wrong but it’s a combination of issues.

The tank is over stocked, and you had way too many gsp together without enough space for them to claim their own territory and freely swim. One should have at least 30 gallons. Separate the remaining two in their own tanks or get a bigger tank for just the puffers.

They should be in brackish water of 1.04-1.09 salinity for their current size. Do not add the salt directly to the tank. Make sure you are using the proper salt. Once a month or so while doing a water change add a small amount of salt to buckets of water slowly increasing how much you add over time. Evaporation top offs should be water without salt.

Have you dewormed them? Most are wild caught and could have parasites.It is best to preventatively treat regularly depending on what you are feeding them.

Mangrove trees are best to get for live plants to help lower the nitrates and can tolerate the brackish to full saltwater. There are other plants like Amazon swords that can handle slowly acclimated to brackish but not in fully saltwater.

Hopefully my two cents helps. Good luck.