Fiberglass Water Tanks ???
I have a east coast 31 with two fibreglass tanks, the hull actually forms part of one side of each tank. Currently the tanks are painted, but the paint is blistering exposing the raw hull beneth it. I have investigated the different methods of fixing it, a couple of methods have been suggested, repaint, build new tanks or ignore it. (I have decided to repaint mainly as this is the cheapest and by far the easiest option.) When the tanks are painted no water should be getting into the fibreglass, so the chances of osmosis is practically zero. (My boat is 23 years old, and so far has only had a few blisters, apparently the chances of problems developing at this age are fairly remote.)
I would be surprised if your tanks do not also have some sort of lining aside from just the fibreglass. You should be able to drain the tanks and inspect them before testing, if you have styrene leaking into the tanks you should be able to see small blisters with brown stuff leaking out, just as you would on the outside of the hull. Styrene is actually a result of uncured resin combining with water, if your tanks are well made it is unlikly to be a problem. There are heaps of good sites with info on osmosis around the net, try http://www.marinesurvey.com/ or http://www.epoxyproducts.com/.