We have two ponds, now 18 months old and looking really good with loads of plants and good water quality . The larger one, about 1500 litre , flows into the smaller 750 one via a waterfall , we have two filters and all is well . However, the 5 fish we started off with have bred like rabbits, and surprisingly, lots survived ! We now have the orignal gang of five ( about 4 inches in length ) and a mix of goldfish and commets , plus about 25 of their offspring, ranging from an inch to two inches in size, plus another 12 in the smaller pond . Just a little worried in case this is too many fish ? They all apear to be doing well , even the bronze carp are changing colour to reds and oranges and many are really beautiful multi coloured fish, thanks for any advice , we dont want a third pond !
The usual rule of thumb for goldfish is 30-50 gallons per adult goldfish. The requirements for a fish go up roughly with the square of length, hence four 5-6" fish roughly equal one full-grown fish, but of course they'll keep growing. I think you're likely to start having problems as your fish grow. And if they continue to reproduce successfully, you're looking at a Malthusian crash.
Besides finding a new home for some of your fish and/or building a bigger pond, you might want to think about adding a predator to eat the eggs and fry. Here in the US, ponders often use sunfish for this purpose, being careful to have only one fish, fish of all one sex, or less fertile hybrids, lest the sunfish themselves overpopulate. I wouldn't recommend these fish outside of their native range, but I'm sure you have something over there that could do the job.