I can imagine the tranquilizing effects of your goldfish pond. Size or volume do not necessarily matter as long as you don't overpopulate the pond and it has a proper depth. The problem of ammonia is a challenging one. Other than changing or recycling the water, you can line the pond when the pH rises to neutralise the ammonia since it is acidic. How about planting some water plants at the bottom to purify the water? Some plants like Elodea and water lily can perform a good job in this regard.
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