South American pufferfish are found in a variety of habitats, from quite soft and acidic rivers like the Rio Negro through to the estuary of the Amazon River. In other words, water chemistry itself is not particularly important. Anything between pH 6-8, 5-20 degrees dH will suit them well. While not common in brackish water habitats, they will do well in low salinity systems up to around SG 1.005.
On the other hand, as with all pufferfish, water quality should be good. By pufferfish standards, South American puffers can be considered hardy, and they are certainly less sensitive to nitrate than things like Tetraodon mbu. But that doesn't mean that they will put up with bad water quality for long. Generous filtration and regular water changes are essential. Taking into consideration their need for strong water current, canister filters (either internal or external) offering water turnover of 6-10 times the volume of the tank per hour are in order. Change 25-50% of the water per week, aiming for less than 20 mg/l nitrate if possible, and certainly no more than 50 mg/l.