i personally wouldnt add 2 in there, a small amount of room will probally cause aggression and one being bullies by the other.i have all mine in single 5 gallon tanks.they all do pretty well and are happy living on their own.any substrate will be fine, gravel or sand as they are going to be no bottom dwellers no need for anything special.i use sand and my puffer seems to be fine with that. as for the plants, i just used any i liked the look of, as my tank was cycled (dwarves need perfect water quality) and it already had live plants in, i just left it, for 1 puffer, a nice rock formation looks good, strong plants are good to have, as are quick growers such as java moss, java fern, riccia, etc.
with 2 i guessing you would need lots of plants and rocks to reduces aggression. live plants help the water quality aswell.
dwarf pufers have lots of personality and need mental stimulation, lots of plants, rocks, decor is good for them, try moving a few objects around as it will explore its new layout.also hide bloodworms by plant bases and in caves so the puffer has to look for them, it keeps them active and busy. adding a few common snails is also good for them, i usally add a handful while im doing a water change every few weeks or so, they hide amongst leaves and rocks and the puffer will eat them as he fined them.
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