This is called the acclimating process, which allows fish to adjust from their old water parameters to the different ones that exist in their new environment. You will need to make sure you have set up and will maintain their new home correctly and followed the care instructions for your particular variety of fish, such as Koi or goldfish (which are the two most popular pond species). This may require doing a bit of research beforehand.
Your fish may need some time to settle and may try to hide, but this is normal. For varieties such as the goldfish, only feed your fish what they can eat in 5 minutes. Start with a palm full and work up from there. In a pond that has plants, fish only need to be fed once or twice a week. The more you feed your fish, the more waste they produce and an overabundance could cause problems in the pond environment. Please stop feeding your fish in the fall when the water temperature falls below 50 degrees and resume feeding them in the spring when the water is above 55 degrees. They may “beg” for food before this happens, but feeding them will cause them more harm than good. Follow these simple steps and you will have healthy, happy fish for years to come.