I've just been a long-time lurker on the boards, but I think I can help here (I use fiberglass quite extensively in the marine industry).
Yes, it will easily be strong enough. Most people well underestimate fiberglass. However, stempy hit the nail on the head. Cost is going to be the major player. A gallon of good polyester resin will run you $40-50. Double that if you want to use epoxy resin. Fiberglass will run you $2-5/yard for CSM and $6-8/yard for biaxial. That may not seem like a lot but when you start trying to figure how much resin you will need you will quickly scrap the idea.
Estimate your tank idea at about 20 square yards. You'll want quite a thick layup. 1 gallon of resin will wet out about 4 sq yards of CSM or 1708. So that's 5 gallons for ONE layer of CSM. You will need, at minimum, 25 gallons of resin... or about $1000. That's resin alone. Forget about your wood costs, windows costs, fiberglass costs, supplies costs, etc.