Exactly what Sorcha?

Does it have shade? What is the temperature of the water? If it has shade in some portion and the water temperature isn't getting much above about 86F and there are no great temperature fluctuations, 2-3 fish in 30 gallons of water should probably be a maximum.

Otherwise, given heat, the amount of nitrogen even 2-3 goldfish will produce and if you don't add fresh water occasionally or do a complete change of water, or get into filters too, you aren't going to have an outdoor fishpond, you are going to make bouillabaisse like I did.