Porcupine puffers in the wild grow to 18 inches on average, which means there is the potential for them to reach this size in aquaria. However saying that they rarely do and 8-10 inches is the norm in artificial surroundings i.e the home aquarium, but the fact remains that even at this size they require at least a 300-400 liter tank for sufficient room. As you they says they are lively little fellows as well as intelligent and therfore require plenty of room to investigate and search for food in. In addition they are messy eaters and will quickly pollute a smaller tank with nitrates and phosphates; a bigger tank will have a bigger filter to cope with this trait.
If you want to give them the best life possible which I am sure you do then a 400 liter aquarium is ideal. 180 liters is far too small for even a juvenile.
Hope this of use
Kindest regards
Joe