Is the spring water cool enough to raise trout? Trout are a great deal of fun.

Also you can always feed the fish regularly by hand, which is always fun. Others here have great luck with that, and in a smaller pond it may be easier to feed train fish. My fish ignore the food, so I gave up.

There are some great experts here on raising fish, I am simply repeating what I read. I believe the challenge in a small pond is maintaining balance between the top predator and its prey.