If a pond is large enough, many species of fish can coexist peacefully, however in a small closed system such as most of our custom-built ponds today, this is not the case.
Understanding that in any body of water there are stratifications in which different life forms are comfortable will help in determining whether different species will be compatible in your pond. For instance, what particular needs will or won’t be met in the limited space allotted?
Comets have a wide range of acceptable parameters, such as pH, temperatures, and even water quality. They need a minimal amount of space and increase in numbers rapidly. Shubunkin and other fancy carp are very comfortable in a goldfish pond with comets. The lionhead and other very special goldfish varieties are best viewed in a tank, so they are not particularly suited to pond life. Their lack of swimming capability would make them easy targets for predators.
Orfes on the other hand need room to run, being fast swimmers and school fish that grow to about 18” at maturity. They prey on slower invertebrates, fish fry and insects. They are not aggressive with other fish in the pond and will not “nip” other inhabitants.
Somewhere in the middle are Koi. Koi are large-growing, high demand fish that need attention to water quality, pH, ammonia, nitrites, nitrates, phosphates, and are sensitive to overcrowding. They are sensitive to parasites and bacteria to which comets are practically immune. They will benefit from doses of salt in the pond for the occasional parasite attacks, but Orfes can die from salt in their water.
What does this mean? I conclude that koi and comets have very different requirements in their living space, so they would do best in their own ponds. Granted, the comets wouldn’t mind living in a koi pond, but the koi won’t do so well in a crowded goldfish pond. Just as the koi will destroy the goldfish pond that contains lovely plants…. And Orfes can coexist with koi if care is taken in medicating the pond and there is sufficient space for all. In fact, they are fun to watch as they dart around in a school of 5-6, not caring what the koi are doing. They are not happy if there is only one.
What about catfish? Many people want to put catfish in their koi ponds, so there are some things to be considered here too. Catfish grow to huge sizes—rapidly! They will eat smaller fish and they do not need clean, clear water. In fact, they prefer the comfort of murky waters where they navigate wonderfully with the sensory guide of their barbells (sense of smell) and hearing. The consensus is that these fish will clean up the uneaten food that settles to the bottom, being the bottom feeders they are, but we fail to realize that anything that eats also poops! It is still in the pond, in one form or another. So you don’t really want to put catfish in with small comets, do you? And given the different water quality preference, you wouldn’t want to pair it up with koi either. I would think of putting catfish together with something like bass, trout and sunnies, if you have the room.
Hobbyists in the southern US have put algae eaters (tropical fish- mainly Plecos) in their ponds and some have written to us about whether these fish are really compatible with their other pond fish. Again, look at the individual needs of each fish. It isn’t whether or not they will fight. It is whether or not they will survive under the same conditions. The plecostomus has a huge sucking mouth, but a very tiny throat, so it isn’t eating any other pond fish. It is an herbivore. If any fish are dying, more than likely their personal needs aren’t being met. Would a larger pond meet them? Perhaps it would, especially if there were enough room for stratifications, different areas with different pH and temperature zones with a variety of food sources accumulating in these “pockets” of life.
Turtles in a pond may be asking for trouble. Pet turtles, water turtles such as Red-eared Sliders which are native to ponds across the country, are fish eaters. They are good swimmers and difficult to recapture once living in a pond. So, before putting your pet turtle into a fish pond, know you may be feeding him your fish.
Frogs are another natural pond inhabitant that we love to see at the pond’s edge. They are the best bug-eaters and evening serenaders! People buy tadpoles and add them to the pond so the frogs will stay around. We worry about how to help them over winter. We love them. But are they really good for our pond? Some of the negative aspects of frog ownership are the bacteria that frogs bring to a pond, the bullfrog’s capacity to eat small fish, and perhaps the neighbors’ complaints if they don’t like the serenading as much as we do.