Is a fibreglass pool for a pond a good idea?
Yes, if you can get one. I say if you can get one, because they are big and bulky, but do not weigh that much (for the size they are) and unless you collect it yourself, (a reasonable sized one will not fit on or in your car) courier companies are reluctant to transport them since they charge by weight, and need a big lorry to carry them. They are expensive to buy, but they will last a lifetime.