Fiberglass goldfish ponds can range from 50-100 gallons right up to the size of swimming pools. (A second hand swimming pool would make an ideal pond, especially if it came with a filter).
Larger ponds are designed for in-ground installation. If you want to use a smaller pond above-ground, make sure it is designed to be filled unsupported. Some fiberglass ponds are made so thin they must be installed in-ground so the surrounding earth supports the sides.
Fiberglass ponds come in simple and irregular shapes. For breeding and raising fry, a simple round, square or oblong shape is the most useful. And you want to be able to fill it above-ground.
Fiberglass ponds come ready to fill with little the owner has to do apart from organizing an overflow.
Fiberglass moldings can have release wax residue on the surface of the gel coat. It can be cleaned off with a wax remover, but make sure there is no wax remover residue left as it is toxic to fish.
Children’s paddling pools make ideal small ponds that can be used for breeding or raising small fry. They are designed to be filled above-ground. Test for toxicity if the pond isn’t sold as a fish pond as some manufacturers add anti-algal chemicals to the plastic.
Rigid plastic ponds have replaced the smaller fiberglass ponds because of cost, but their size is limited. They aren't big or deep enough to be used over winter in colder areas. Some of the designs are more aimed at children than goldfish keepers.
The ponds themselves are maintenance free and generally quite tough. The rigid plastic ponds can degrade over time through exposure to sunlight, although the black colored ones are usually UV stable.
Some of the smaller ponds are the same size as larger aquariums, but being located outside they are exposed to greater climatic changes and require more maintenance. Some options to consider when setting up your pond to make maintenance easier are covered on the Goldfish Ponds Setup page.