Wild fish often live with parasites, and dropping them into a contained pond can allow these parasites to spread and quickly wipe out your cherished pet koi. Keeping wild fish in a quarantine for ~40 days is the best way to minimize the risk of transferring any parasites and disease to your pond. To learn more about that quarantine process, click here.
As predators, bass will eat whatever moves and fits in their mouth; on occasion they will attempt to eat things much bigger than their mouth. If your koi are twice the size of the bass, that should keep them safe from being eaten, but they are still vulnerable to aggressive bass defending their territory. Giving the bass plenty of options for hiding spots will maximize their comfort and decrease the likelihood of territorial aggression. Water lilies, rock caves, coves, driftwood, or other structures within the water will create shade and cover for the bass, and allow them refuge from disruptive koi.
A habitat that meets the needs of bass is a great start, but there are some other concerns to keeping this game fish in a small pond, such as oxygenating the water and providing sufficient food.
Bass need well-oxygenated water, unlike koi which can do fine in poor conditions. Consider the level of aeration needed for koi and do at least 50% more aeration for bass. (An additional waterfall or a wetlands area with it’s own dedicated pump will usually suffice to provide adequate aeration for bass.)
Bass have different needs than koi. A pond built for koi can probably support a few bass for the summer, but to keep them over winter the pond will have greater requirements for size, structure, and depth. A pond that is not well-suited for bass to comfortably over-winter in its depths may still offer seasonal enjoyment by keeping them as warm-weather guests in your pond and releasing them in the autumn. Bass are intelligent fish and are interesting to observe; if you’re an angler you can learn a lot about their feeding habits from keeping them in your pond. That knowledge may one day make the difference between having a successful day on the water and going home skunked.
With these considerations in mind, it is easy to see why keeping ‘pet’ bass is uncommon, but with planning, determination, and realistic expectations it can be a rewarding and educational experience.