The ones you stock can grow quite large, but they will reproduce, and the offspring will regress back toward smaller, less desirable fish. Also, if you use a blue dye to color your pond water for weed control, don't be surprised if your fish grow slowly. The dye retards the development of an adequate food chain. An adequate oxygen level in your pond water not only keeps your fish healthy but also reduces odors and cloudiness by oxidizing waste products. Under warm conditions, water holds less oxygen but aquatic organisms have a high oxygen need.
Cold water holds much more oxygen but the demand by organisms is lower.
Most pond fish need at least 5 ppm oxygen to stay healthy during the summer. Test kits are available which let you check the oxygen level in your pond, but some simple observations usually can tell you if the level is high enough. Crayfish will climb the stalks of cattails or other plants to the surface when oxygen is low. If the water gives off a sulfurous or rotten smell, it signals that decaying vegetation is consuming all available oxygen. Adequate oxygen can be maintained by removing excess vegetation from the water and by keeping pond banks fairly open, allowing wind to circulate the water.