My bro is planning a pond for his garden and needs some good advice, so I said I would ask you lot He wants to keep shubunkins and wants to know some general advice about how deep the pond should be and how many gallons to allow per fish and any other tips pond owners have. Thanks in advance! :)
Pond should be no less than 18 inches deep to allow safe overwintering, 24 inches is preferable, gives a safety margin. 8 to 10 gallon for each fish is recommended, some people prefer to use the surface area rule ie. 24 square inches of surface area to every inch of goldfish , excluding the tail fin.
If your area has bad winters, I would go deeper. Deeper water also offer more protection against prediators. The pump/filter setup should be able to completely turn over the total volume of the pond within 2 hrs, 1 hr is better. You should allow more than 10gal per goldie as they will grow bigger in a pond. 25gal per fish would be a good idea.
As stated 18" is considered the minimum depth in the UK, however I would go deeper, anything under 2ft deep is heron wade-able. If the pond has a small surface area it may be simpler to dig the entire pond at full depth and then create shelves with plastic flat pack shelving units. A small tiered pond with a deep deep can make liner folding 'fun'