You don't need anything else to make it into a marine tank, but it is extremely difficult to turn a 10g (or was it 15g?) into marine tank under your circumstances. Here is why:
First your bioload will be too much and you will eventually have to do multiple water changes PER WEEK for good water quality. You will benefit with a skimmer and use a HOB
refugium for macroalgae because they help keep the water cleaner in between water changes.
Second, you need to go buy an auto top off system. The salinity level will change every day or other day due to evaporation. You want to keep the salinity level as consistent as possible. With a 40g, you see less changes is salinity level and water quality is better in a given week.
The current tank is too small and your puffer will likely get stunted. Again, I highly suggest keeping your little puff in light-brackish water until you get a new tank set up. You'll save more money in water changes.