This is a very good question and one that more pond owners should ask themselves before making livestock purchases. It is easy to forget that something as humble and common as the goldfish can grow to 30cm in length and live for 25 years or more. There are a few thoughts to consider when stocking your pond and one or two options that you may not have considered.
Firstly, if you want to keep a natural wildlife pond, you should think hard about adding fish. Not all native wildlife will avoid ponds with fish but some of the more glamorous, such as newts, will. By comparison, toads, having toxic tadpoles, prefer large ponds with fish as these contain fewer insect predators. If you want to introduce fish then sticklebacks are a good choice for small ponds due to their fascinating behaviour, but they can effect insect and amphibian populations. For larger ponds, golden rudd are a great choice, as in the right conditions, they can breed at a rate that can even keep pace with predation by herons and they also eat duckweed. While you may still get things like dragonflies or frogs in a large natural wildlife pond with the odd goldfish in it, it’s worth remembering that once a fish has gone in, it can be very difficult to get it out again.
Although rarely seen nowadays, coldwater catfish are normally predators that can grow to the size of carp in the case of channel catfish (Ictalurus) and should only be added to pools with equally large species. Legislation has reduced the selection of oddball pond fish and on the whole this is a good thing. If you want to keep your pond well-planted, then it’s best to avoid larger carp such as koi. Goldfish will root around in planted areas, but Koi are very messy and will happily dig up even the most established of plants with their rooting activity. Grass carp have an 'all you can eat' approach when it comes to planted ponds and will happily eat any aquatic vegetation. This behaviour can be harnessed to control blanket weed in unplanted ponds, such as those containing large koi. As far as aquatic pets go, nothing beats large carp but make sure you have plenty of room to allow them to flourish.
Finally, you have more specialist fish such as Sterlet and Sturgeon. Both these animals have very specific needs that must be considered and satisfied before they can thrive. As these are large, ungainly fish that like cold water, they need plenty of room and will thrive in the sort of conditions that suit koi. This means a large water volume, and to keep things cool, you must have a deep pond - 3ft is a good working minimum. Next, you will have to invest in specific foods for the sterlet or sturgeon. Due to their anatomy, these fish have a small mouth and so can only feed successfully on food that is beneath them, although hungry specimens can feed upside-down with a little effort! You will need high protein, sterlet/sturgeon-specific sinking pellets and you might also get a better feeding response if your pond has a flattened bottom, as this aids in successful feeding behaviour.
Sterlet and Sturgeon have high oxygen demands, so you may need to consider additional aeration for your pond, especially in summer months when the temperature rises and oxygen levels decrease. As these fishes are clumsy swimmers, you must ensure that you do not have any major or significant blanket weed problems. Surprisingly, even large sturgeon can become tangled in the long threads of a blanket weed infestation and subsequently die. Finally, you must consider the size these animals will reach. Sterlet are the smaller variety, growing up to a metre, however Sturgeon can grow considerably larger and can reach two metres or more. Such a regal specimen deserves a fittingly king-sized pond to cruise around in.
So, after looking at the options it's clear why goldfish and their varieties such as shubunkin, sarasa and canary forms are such popular choices. They're colourful (especially when mixed), not too big, not too demanding and when trained to associate their owner with food, almost as friendly as carp