Building a small fish pond along the south wall in the courtyard facing south has any influence on Feng Shui?

Of course, the bigger the pool, the greater the impact. The impact is mainly:

If you dig a new pond, the atmosphere will be destroyed.

In case the water is in Wangfang or Yaosha, it may be fierce because of the water.

So you'd better find a feng shui master to test it.