If you're talking about a Star Base located at a planet, then in order for that Star Base to get Hydrogen or Caslon in its cargo storage, freighters will have to bring it there. I think that that will only happen if the Star Base has a Commerce Center. I think...
Alternatively, you can make a Star Base with 8000 cargo space, some docking bays, and 1-2 gas extractors (you need 40 extraction to max it). Place one of these on a world with only Hydrogen or only Caslon (as you need), and when built, it will fill its tanks up with the chosen gas and act as a refueling base. The 8000 cargo space will give you room for 32000 gas, which is 1000 over the hardcoded storage cap.
If you mine a 50% Hydrogen planet, the station will reach maximum cargo capacity for the chosen fuel in 10 game months. A 75% Hydrogen planet will fill to capacity in 6.7 months. Therefore it might be a good idea to build a couple of these things on the same planet, to speed up the overall rate you restock your fuel stores.