There are some Fiberglass resins that are safe and others that might not be. Might be worth looking up the plywood tank manual that friendlies put out.
Biggest thing to think about when doing wood, is termites. I used to do plywood and liner beds but I had some termites and carpenter ants install themselves some new plumbing which kinda sucked. Now I'm using lots of 100 gallon rubbermaid stock tanks for gravel beds.
Fiberglass is safe to use. I have used it for years now. There are two types of resins generally used. Polyester resin is cheaper and weaker than the more expensive and stronger epoxy resin. The polyester resin is fine for this use though.4x4 gavel beds can easily and cost effectively made using a single sheet of plywood cloth and resin. It is best to glass the entire 4x8 sheet first with a 6 ounce cloth. Then from the 4x8 sheet cut 4 rips that are 12" high and 4' long that will be attached as sides. Then use 6 inch by 6 ounce roving to glass the sides together and seal. The wood can be pretreated with copper wood preservative to deter bugs then the outside can be heavily painted to further prevent bugs. PVC fittings can be easily glassed in eliminating expensive bulkhead fittings. Fiber glassed beds will last a long time if built properly.
Plywood that is lined with pond liner is prone to puncture depending on the media being used. Heavy or sharp media can take time to eventually puncture a liner and cause slow leaks. Care has to be used when digging in the media and using shovels or other tools so the liner does not get punctured.
As Chris says, liner can get punctured and fittings through liner will be more prone to leaks as roots work on them and the space between the liner and plywood makes a place where bugs (termites/carpenter ants) might set up home and cause you problems later.
If you do use liner with wood, make sure to treat the wood (and make sure water can't drip off it into your system) And use the heavy duty EPDM Firestone pond guard liner since that is the only one I would trust against gravel where you can't see the outside of the liner to keep and eye on it for leaks.
Thanks so very much for the information. I knew there were different fiberglasses now I know which one to go find. I plan on putting my GBs on metal stands so this should elimate the bug problem, or make it controable. Thanks so much again.LF