When discussing fibreglass, a distinction must be made between moulded, drop-in fiberglass shells, and fibreglass linings that are applied to concrete shells when the marbelite coating starts to erode.
Drop in shells can be converted to NSPs, but be warned that the structure cannot support the weight of dividing walls.
The great advantage of fibreglass linings is that it comes as a liquid and can therefore be applied to any shape.
However, fiberglass is not cheap and can crack under the weight of gravel beds or rock. Fibreglass also has a limited lifespan and can get very slippery, especially in NSPs where there is always some growth of biofilm and microbes on the shell interior.