I would advise to empty the cannister and throw some LR rubble in it as the usual FW media just tends to become a nitrate trap. Alternatively it becomes a handy place to store carbon, phosphate removers, and other chemical filtration.
If the base rocks are dead LR then that should be a good idea, though if you fancy something a little more DIY (and have the time) you can always make some LR yourself.
All you need is some cement and some bulk (crushed shells from clams, oysters and the likes works well).
Mix the cement and crushed cell at a rate of about 1:5 (cement:bulk) and add water until it just holds itself in a ball. Then leave it to set in a container full of sand (after moulding the shape you want) and after a day or so rinse the sand off.
Then you just need to leave it in a bucket (or ideally in the toilet cistern so it gets frequent waterchanges) until it stops affecting the pH of water. At that point you can use it as good base rock to be colonised by your LR.
One thing you haven't mentioned is the flow rate. You will want a minimum of 370 (10x) gph and should look for 720 (20x) gph.
The tank is small, so some would not recommend a skimmer, but since the puffer will be a fairly messy predatory fish I would say a good skimmer will help you no end (that and most of a clean up crew would just get eaten by it). I would probably get one rated for 60-70 gallons. When it comes to preds, you can't overskim.
