So you just finished installing your new backyard pond, and now you want to add some fish. Great idea! They enhance the beauty of your pond, and can serve as natural mosquito-killers in the summer. (If you've ever had a pond, or even just an old garbage can that collected water, you know how icky all those skeeter larvae can be.) How do you go about stocking your pond with life? Let's get started. (If you haven't read up on basic care for fish, please do that first.)
I'm going to assume you already have plants in your pond. If you don't, you basically have a hole in the ground with water in it, and need to get back to work. For aesthetic purposes, most people plant several different plants that will grow above the water as well as underneath. A few sprawling plants on the surface will shade the pond and cut down on the algae population. The stems and roots below the surface will provide plenty of hiding spots for your fish once you get them. Plants that grow only underwater will provide your fish with plenty of oxygen and will help keep nitrogen levels low. Check out your local nursery, home improvement store, or sometimes even your pet store for plants that will do well in your pond. If you decide to try to grow some plants you already own in your pond, be sure you know which plants are toxic and/or appetizing to goldfish.
Another thing to think about before putting fish in your pond is chemical use. Do you use potentially harmful chemicals to clean your pond or fertilize your plants? They'll have to go, and you should empty your pond and clean it to remove all traces of the chemicals. From now on when you fill or top off your pond, use clean water from your kitchen sink and a dechlorinator. Though it seems obvious to do so, please don't fill your pond with your garden hose. Most hoses are made of plastic that leech toxins into the water, and it should not be used in a pond with any animal living in it. If you have a pond so large that it simply doesn't allow you to fill it from your sink, you can always take your garden hose off the faucet, or at least let the water filter for a couple of days. Ponds this size should definitely have a pump and filter setup.
The size of your pond determines the size and number fish you should buy. As far as I know, only goldfish or koi are used in these types of ponds, due to the fact that most any other fish needs warmer water than a pond would provide. (If you know otherwise, let me know.) And though it sounds it a little harsh to say, you may want to start out with some small, inexpensive goldfish until you make sure you have the conditions right to keep your fish alive. For the most part, though, you won't find a hardier fish than a goldfish. When calculating how many fish you can get, the current rule for indoor aquariums is one inch of fish per gallon of water. In outdoor ponds with good plant support, you can probably stretch this by a fish or two. How many gallons of water are in your pond? An easy, no-math way to tell is to fill your pond with a bucket (from the tap, not hose!), and keep track of the gallons using the marks on the bucket.
After you buy your fish, acclimate them to the water just like you would an aquarium, and let them go. Now is the time when you should closely observe what goes on in your pond. The environment will be changing quickly over the next week or so, and while you should address any major problems, it's also very important to let the pond and its new inhabitants balance things out on their own. In case they need a little help, here are some problems you may encounter and how to solve them.
In part, your fish will be able to feed on the regular supply of insects that will drop into the water. But there probably won't be enough unless it's mosquito larvae, and there may be some concern that insects will carry pesticide residues. You should always keep some fish food on hand regardless. Pet and garden supply stores will have goldfish and koi food, usually in pellet form, and instructions on feeding. Drop a few pieces in to your fish. If they devour it quickly (after they first notice it), they're probably being underfed, and you should continue feeding them regularly. Otherwise, just use it as a weekly supplement.
If your fish eat, they're going to poop. And fish poop, as well as excess food, will break down in the water and build up nitrogen levels quickly. This is deadly for fish. You can buy water-testing kits at pet stores, though many will do the test for you for free. There are a few products made that claim to reduce nitrogen in the water, but I feel the best method is the natural way. How do you do that? In addition to plants, as I mentioned above, believe it or not, buy more fish. Not too many, just one or two bottom feeders that can help eat excess food that has sunk to the bottom and will otherwise just decompose and pollute the water. Try these solutions, test the water often, and soon you should find that nitrogen levels have balanced themselves out to a safe level. To keep it this way, don't overfeed, and if your fish reproduce, you should remove them before your pond gets overcrowded.
If you find that your pond has become overgrown with algae, there are several solutions you can try. First, you may be tempted to empty the whole pond and start over. Don't. It takes a long time to achieve balance in an environment, and destroying what's been done so far is not the solution and would be extremely stressful (possibly fatal) for your plants and fish. First, look at your plant situation. Plants that cover the surface of the water (like a lily pad) will block light from reaching the water, thus controlling the amount of algae that can grow. Do you need more of these plants, maybe? Also consider buying an algae eater or snail to put in your pond. These are very efficient little cleaners that really should be in any pond or aquarium big enough for them. Warning about algae eaters: Those weird-looking algae eaters called plecos get HUGE (sometimes up to 12"). Don't buy one of those unless you have a good-sized pond (the size of a bathtub or larger). The tiny catfish are good for smaller ponds. Warning about snails: they can reproduce in large numbers, so you should probably only buy one. The large yellow Mystery Snails are very common in pet stores and do well in captive environments. By the way, if you're wondering how Mystery Snails got their name, apparently that's the mystery -- no one knows what's so mysterious about them. Interesting story, huh?
If you put out bait, you're bound to catch something eventually. And plenty of animals will soon become interested in your newly stocked pond after they realize it's there (if they haven't already been coming for the water alone). Cats are an obvious enemy of your new fish, but also raccoons, opossums, birds, and some snakes may prey on your fish and snails. There's not a whole lot you can do about this, short of putting a large wire screen over your pond, but talk about ugly. But if you're still looking for a solution, fret not. The first solution that comes to mind is to buy bigger fish. Some koi can get up to 8 inches long or longer -- not as easy a catch as the little ones. If this doesn't deter your neighbor's monster cats, try some new plants around your pond. I'm thinking something sharp and spiked like pineapple tops or cactus, if your climate allows.
As a last resort, you can try capturing nuisance wildlife for relocation, but please don't. The best solution is to enjoy all the animals in your yard, not just the fish. If a garter snake comes along and grabs one of your new goldfish fry, is it really going to hurt anything? And it may cost you another dollar or so to replace that snail stolen in the night, but opossums have to eat, too. Some people will put this beautiful water feature in their yard, probably show it off to everyone that visits their house, yet want to keep all other wildlife far, far away. It's a shame. (I know I'm getting a little preachy, and I apologize. But I do feel strongly about this.)
Still have fish left? Good. Now, unless you're lucky enough to live in a place where it's warm all year round, you're going to have to decide what to do with your pond pets when winter comes. My personal option would be to remove my fish and keep them in an indoor aquarium until spring. A kiddie pool with a pump attached would work if you have large koi, and can be temporarily set up in a garage or basement. If you decide you would rather keep your fish in the pond all year, the only requirement is that your pond is at least 18 inches deep (or below the frost line). This will allow enough room beneath any ice for your fish to hibernate. You'll also need to keep a small hole open in the ice to allow for an exchange of gases from the water. Otherwise, the toxic gases will build up beneath the ice and your fish will die. Don't count on your fish freezing in the ice and then magically thawing out good as new in the spring; I don't care what the cartoons say, I wouldn't do that to my fish. Before the big freeze, slow the feeding schedule, and there is no need to continue after the water has become frozen. In the spring, as temperatures warm up, offer your fish food gradually until you're back on schedule.