You can get a 10 gallon tank for around $13 at Walmart. Would that be possible? Do you have a large plastic bin you could use as a temporary home? I used a 20 gallon plastic storage container as a QT tank for plants and such. Could you divide the 10 gallon you have for now? Take some plastic mesh and stretch it across the tank to divide it into two sections. The puffer will kill the other fish by taking bites out of them or just harassing them to death. If none of that is possible you may want to take him back to the pet store or take the other fish back.