Fuel

> This has probably been discussed on the list before, I joined it, but

> I thought that it would probably be skimmed from gas giants, ala
streamlined
> tankers/fleet auxiliaries out there, & I've never seen a mention of

Depends on your chosen genre.
For Star Trek, dilithium crystals and anti-matter.
For Babylon 5, ____________________ [your answer here].
For GZG Full Thrust, I think it's unspecified. But DSII allows Fusion
Generation Plants which use fusion power. So, perhaps the answer is any source
of hydrogen, whether as water (then cracked into hydrogen and oxygen) or
hydrogen gas from gas giants.
    This gives another possible objective in attacking/invading star
systems. Controlling the star system's gas giants would allow easy refueling
as in Traveller. Except if you're invading a colony world with plenty of
water...