You can have plants and smooth rocks in tanks with puffer fish if you like. Some of the bigger puffers eat bits of plants but most ignore them. You can have ornaments in the tank too and some puffers will sleep inside ship wrecks or caves made from rocks or ceramic flower pots.
Make sure the fish is fed a variety of food ranging from fish, prawn/ shrimp, mussel (in its shell), squid or octopus, worms, snails, non toxic insects (that have been caught without bug spray), fish pellets, basically anything. Feeding them a varied diet will help prevent malnutrition and hunger strikes if the fish doesn't get its favourite food every day.
If you catch small crabs at the beach you can give them to the puffer but try to kill the crab first. Mussels from the beach are fine but don't put too many in the tank because they die in freshwater and will pollute the tank and kill the fish. Freshwater mussels are better because they can live in the tank.
You can keep pond and Ramshorn snails in a spare tank and put some in the puffer tank each week for the fish to eat.
I don't normally suggest feeding lives foods unless you have to. Most puffers will take dead foods just as readily as live foods, and being torn apart by a puffer fish is pretty painful and not something any fish, shrimp, crab or organism should have to go through. So try to feed dead foods rather than live.
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Do big (75%) water changes and gravel clean the substrate every week. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Puffer fish, cow fish, box fish and all their relatives produce poisonous chemicals that can be released into the water if they are stressed or die. In an aquarium this poison will kill the puffer fish and any other aquatic organism in the tank. So if the fish looks unwell or stressed, do a big (75%) water change immediately and add some carbon to the filter.