I have seen them up to 30 inches in the wild. This fish will outgrow your system. You might try gut loaded live ghost shrimp or tiny live crabs if you can find them, small crayfish, raw blue crab legs; do not remove the shells, the shells are needed for proper wear on the teeth. Also try marine snails, no bigger than the puffer's eye. Some have written that snails are actually essential to the health of puffers.
The thing with puffers; they have a BIG problem with diet in marine systems; it's usually the same thing, they stop feeding... which makes me think they are very misunderstood in the hobby and perhaps most aquarists should not even attempt to keep them long term. A VARIED diet including crunchables is essential. More research on the diet is in order here; try fresh clams or mussels in the shell; open them a little then freeze for at least a week. Then thaw one and drop it in shell and all. These messy feedings demand the most efficient skimmer. Many aquarists find that a few months of this creates soaring nitrate/DOC issues.
IMO, puffers often harbor internal parasites, which are resisted for a while, but then "catch up" with the puffer after a while, particularly if there are other stress factors at work like water quality, limited diet, etc. There are also reports of a "lock jaw" condition due to possible thyroid problems, some references to iodine treatments. IMO, these fish do not belong in the average aquarium.