Septic is anaerobic by definition, Larry
The type of aerobic system you describe isn't a septic tank, it's a small treatment system, capable of recycling waste water. Even better than the type you describe are those that rely solely on filtration and plant life--no chlorinator. Where water for non-potable use--grounds watering, car washing, even toilet flushing, is in short supply, that type of system is gaining favor in office parks, golf courses etc where large amounts of waste water would otherwise just be lost. One of the first businesses to install one was Ben & Jerry's Ice Cream in VT (NH?)...it's all enclosed and absolutely beautiful...looks like a tropical rainforest in a greenhouse. A septic tank, otoh, is an anaerobic system that simply collects and stores waste, and drains off liquids into the soil. A boat holding tank is neither a septic tank nor a treatment system...it's just a storage tank, but subject to a LOT of stresses that systems on land are not. Boats take a terrible beating, and they flex...just coming about hard flexes every inch of a boat. Because of those stresses, fiberglass is problematic everywhere on a boat...blisters, cracks and crazes in gelcoat, delamination, are just a few of the problems. Stanchions, hardware and thru-hulls require re-bedding and re-sealing periodically to prevent not only water leaks, but water intrusion into the fiberglass. The archives here are filled with assorted fiberglass problems. Fiberglass tanks are subject to the same problems as the rest of the boat--tank fittings are no different than any other thru-hull, neither is epoxy or gelcoat any different in a tank than anywhere else on a boat.Poly tanks, otoh, are more shock resistant, more flexible (within limits!), and non-corrosive. That doesn't mean that poly is the best material for ALL applications, aboard and ashore...only that it's the best tank material for boats.As for glassing over a failing metal tank...metal doesn't dissolve, it disintegrates into rusty chunks and "slices" that would clog the plumbing and destroy a macerator. And with this, I have spoken my piece on this subject unless someone else has a question.