Before adding fish to your water garden, check the water quality with an inexpensive kit available at most pet shops and home-improvement centers. If your water is high in ammonia, chlorine, or chloramines, use a product such as Amquel to neutralize the chemicals. Also check your pond’s pH level. If it’s much beyond the 7.0 range in either direction, add some Neutral Regulator to bring the pH back to an acceptable level, somewhere between 6.8 and 7.2.

Remember that, as a general rule, the smaller the pond, the more fluctuations in water chemistry you’re going to have, meaning the more tolerant your fish must be. Water heats up and cools down with ambient (surrounding) temperatures; small ponds of 50 – 100 gallons fluctuate more with cool mornings and hot afternoons.

A pond that is 3 feet deep holding several hundred gallons or more, on the other hand, can house fish with a narrow tolerance for temperature and chemical change. That makes larger ponds ideal for raising koi, which need plenty of space to roam and water at least 2 feet deep, even though koi often spend much of their time at the pond’s surface—another bonus for keeping koi in the pond.

Selecting New Fish
When picking out new fish for your pond, try to find young, healthy fish from 3 to 5 inches in length. They should have bright eyes, a sturdy body, fully extended fins and tails, and a lively inquisitive habit. They should have no damaged or missing scales and show no unnatural spotting or lesions. You can easily detect all these things simply by watching the fish swim around the tank for a while. If you find one that seems particularly interested in watching you, you can bet that it's (a.) more intelligent than mosquito larvae, and (b.) going to be more fun to relate to than your Aunt Edna.

Just remember that, no matter how many fish you buy, don’t place small fish in with larger koi, who relish just about anything that will fit into their mouths! That’s one good reason to have a second small pond or fish container up and running to serve as a nursery for small fry until they’re large enough to join the gang.

We use a wine barrel fitted with a mechanical filter for raising small goldfish to the point where they can be moved to a larger pond and released with the koi. The barrel can also be pressed into service as a hospital tank for our sick fish, if necessary.

Buying Fish
When you buy your fish, you’ll probably receive them in a small plastic bag half-filled with water. Get the fish home as soon as possible. Those bags hold only so much oxygen. (How would you like to take a long trip somewhere with a plastic bag over your head?)

Once you get the fish home, make several quarter-inch slits in the upper half of the bag, and float the sealed bag in the pond for 15 minutes. That will allow for both the slow equalization of temperatures between pond and bag and the mixing of some pond water into the bag.

If you’re adding new fish to an existing colony, or if you have any suspicions whatsoever about the health of the fish you’ve just bought, think about quarantining them for 10 days to 2 weeks in a small aquarium. In that way, if they show any signs of disease or parasites, they will be segregated from your main school, and you’ll have an easier time observing and treating them without running the risk of infecting the others.

After floating the bag for 15 minutes, acclimate the fish to the pond by adding three or four handfuls of pond water to the bag every 5 minutes. Repeat several times. When the time is right, open the bag and gently turn it on its side, allowing the fish to swim out of the bag and into their new home on their own.

If your fish are hardy (hardiness varies, even among goldfish, although the single-tailed varieties are far and away the most durable), they can over-winter in your pond, so long as there’s enough open water at all times for oxygen to enter and carbon dioxide to escape. Usually, the area below a waterfall will remain open—another good reason for installing at least one spillway in your pond. If necessary, an electric water heater will also help keep a section of the pond surface thawed.

Remember not to place too many fish into your pond. That could result in deteriorated water quality, stress, and disease. To find the number of fish your garden pool can safely accommodate, figure the total surface area of the water feature in which fish will be present. (Surface area is determined by multiplying the length of your pond by its width.) Don't include in your calculations areas filled with marginal plants, but do include areas covered by floating plants.

As a rule of thumb, provide each inch of fish length with from 6 to 12 square surface inches of water. Koi, as you might have already supposed, require more space—much more. For them, plan on 20 square feet for every fish, regardless of size. If you’re unsure of your calculations, always err on the side of too much, rather than too little, space per fish