Yes, you can fix a plastic toilet tank using fiberglass. First, you need to find the crack in the plastic toilet tank and neaten up the rough edges. Second, you need to cut a piece of fiberglass roughly an inch or two bigger than the gap on either side of the tear. Third, you need to mix resin with the catalyst that is provided in the pack and apply the resin mixture around the tear. Following this, you will need to place the fiberglass over the crack and add another layer of resin to ensure it is sealed thoroughly. Finally, leave it to dry for 24 hours before using the tank again.