An GRP tank will continue to leach monomers of styrene for the life of
the tank. You could extract the remaining unpolymerized species with
super toxics like methylene chloride ... but then you'd have a
humongous disposal problem and a tank containing a potent carcinogen.

That you have a 'plastic taste' means that something in your system is
leaching chemicals and is probably not in conformance to specs for
potable water components. You can do a websearch for NSF (National
Sanitation Foundation) approved components (or better yet move up three
notches to FDA regs.). When choosing potable water system componentry
make sure that the object at least complies to NSF regulations (for
leachables, etc.).

If possible, replace your tankage with either Stainless or UHMW
Polyethylene. If the present tank can be cut open and or removed,
consider to purchase pre-laminated panels of UHMWPE facing over Epoxy
substrate and insert/epoxy to the walls. Get a plastics welder to run a
polyethylene bead at the seams to seal the water from the epoxy
substrate.

A poor alternative would be to use a carbon packed (using FDA/USP grade
carbon) 'filter' but then there would be no way to know when to change
..... other than a crude 'taste; test that would indicate that youve
been drinking styrene monomer, etc. for a long period before it
'breaks-through' the filter and your taste buds finally kick in to tell
you thats something is not right. . A carbon absorbing filter should
be drained completely during long term non-use to prevent growth of
microorganism that use the carbon as a nutrient source. If you go this
route if possible select a carbon system that is made from 'extruded'
carbon ( a micro-sponge made from carbon), as a loose pack of carbon
will eventually break up into very small carbon particles (carrying the
absorbed species into your gastro-intestinal tract, etc.).