Can you drain a fiberglass in ground swimming pool?

No!!! Under no circumstances should you drain a fiberglass in-ground pool. Unlike the standard in-ground pool, the fiberglass pool base is unreinforced concrete or other hard surface material that was applied directly to the soil in a thin layer. It is only there to allow the fiberglass to be sprayed on and form a hard shell. The weight of the water is what holds the fiberglass in place. If you drain the pool without refilling it immediately, you will allow external ground pressures and/or ground water to buckle the sides or bottom and you will have the fiberglass completely redone.