"Fiberglass" tank = plausible? They're commonplace actually and have been for years lol - boats have had 'em foreveh
There are three main types of resins used when doing layups - vinylester and polyester are by far the most common, but I wouldn't recommend them for this application given the tank's location and mounting. Epoxy resins are far tougher - but all
are suffering degradation problems associated with today's rising use of ethanol, so regardless of what type resin is used - you'll need to line the tank.
Vinylester resins and polyester resins are less expensive but have a few bad habits like delaminating/blistering, photosensitivity, and difficulty in bonding dissimilar materials. Polyester resin is the cheapest resin available and offers the poorest adhesion, has the highest water absorption, highest shrinkage rates, and fractures easily.
Epoxy resin is a bit more difficult to work with and costs a bit more, but is incredibly strong and bonds to a wide range of dissimilar materials (like carbon fiber and kevlar cloth, which I would recommend for this) - I would consider nothing else than epoxy for the application. I also choose to use "slow" hardener BTW, as all epoxies really should cure overnight it allows me more working time before it kicks. Yet if you're considering a "production run" of tanks - you might want to try "medium" or even "fast", while you still shouldn't tool the "cured" epoxy till the next day, it's faster set allows more layers to be applied in less time.
The "industry standard" epoxy resin would be without a doubt West Systems. Pricey, I sometimes think it a touch over-rated... While it's really, really
good stuff they've simply dominated the market;
WEST SYSTEM Epoxy
A relative "newcomer" MAS epoxies isn't as common or well known, I've used MAS extensively in a wide range of various applications both structural and as bulk filler and it's without a doubt equivalent to West Systems, if not slightly superior in some minor ways, best yet - it costs less;
Epoxy - MAS Epoxies
As sexy as carbon fiber and kevlar might look - all resins are subject to UV degradation so painting the tank is a must BTW, while you can use the less costly fiberglass - check for compatibility with epoxy as some are impregnated with oily starch material (helps w/manufacturing process) and are strictly for vinyl/poly resins - you want to be sure you're getting cloth that is proper for the resin you're using.