There is a huge amount of complicated ‘advice’ about breeding goldfish – especially food for the very young fish (aka fry).
The truth is, unless you want to breed as a business, it’s really easy:
Your top priority is to keep your main pond (#1) healthy. The commonest problem with newcomers to fish keeping is over feeding. An established pond (12 months +) will produce more food than your fish need, most of the year.
Nutrients are continually finding their way into the pond through visiting birds and insects (the latter being caught and eaten!), so the “closed ecosystem” of the garden pond more often needs help to reduce its fertility (see ‘filter’ below).
You need a bird and cat proof cover (e.g. netting) to protect the fish; also a pump and filter. A good filter contains porous stone which has a large surface area on which beneficial bacteria thrive. These convert toxic ammonia (from fish droppings) in to toxic nitrite, then into non-toxic nitrate (which is fertiliser).
The nitrate causes plants and algae to grow more quickly, and a pad of microfilter can be added to your pond filter to remove algae and other fine particles. When you first start using microfilter the gunk it collects will likely be brown. This is dead algae (for example) and fish do better without living in this 24 hours a day. It took a few months to clear this when I first added microfilter to my pond filter.
Using ultra violet light to kill algae makes dead algae! Dumping this on the bottom of your pond makes little sense. Better to have live algae and manage it … the reason being:
Green water is healthy, in moderation, and the food of choice for all the microscopic plants , animals, algae that thrive in a healthy pond.
It is the best food for daphnia (aka water fleas, but they aren’t fleas!), and daphnia are the best animal protein for baby goldfish as soon as they can yawn wide enough …
N.B. Extra ponds, if made of containers, need to be buried in the garden to ensure a good thermal contact with the soil. This helps to stabilise the water temperature. A tank of water in full sun will experience suhc rapid fluctuations of temperature on a summer’s day that fish, and even the plants will die.
It is fun hatch baby goldfish indoors in an aquarium, but if the weather is okay, (pond temperature > 10 C) they do better in pond #3.
Goldfish spawn onto plants. If you are short of pondweed, then place willow twigs in pond #1, ans after the fish have spawned, move the twigs to pond #3.
The best food, for newly hatched fry is green pond water! You can’t buy this at a pricey garden centre, but ti is free from you pond!
The best food for daphnia (which are the best food supplement for goldfish) is green pond water!
When fry reach about 1.5 cm they can’t eat ‘adult’ daphnia, but they can manage their tiny offspring.
As your fry / bambinos grow, add more daphnia.
If you smaller ponds don’t have a pump and filter (mine don’t) add fresh pond water regularly to dilute the fishes effluent. You can use such water on your veg and flowers and mine seem to appreciate it!
N.B. Test for NO2 regularly to check your water-replacement is adequate.
No matter how successful the breeding in ponds proves, it is interesting to grow some indoors in an aquarium – handy for you magnifying glass, camera ,etc … especially if you suffer from mad professor syndrome 🙂
P.S. Once you know what a filter involves, you can make your own. If there is space for a large gravity fed filter, banks of gravel can be used to grow plants, using the fertility from the pond water.
I also have a grass carp which sadly does not breed readily in a pond.