I have seen many discussions on Pond Boss about avoiding getting tannins into your pond. However, I do not recall a thread about getting rid of tannins once they have become a part of the pond water.

Tannins are large organic molecules (since they come from plants) and the chemistry gets very complicated very quickly.

I am speculating - but many large organic molecules are susceptible to oxidation which breaks them down into smaller component products. I am not aware of any break-down products that are worse than the original tannins, but I am NOT an expert.

You might try a simple experiment using a small aquarium bubbler. Put samples of your pond water into two jars. Put them both in a dark place and run the bubbler in one jar for 24 hours. Then compare the color of the two jars. If the bubbler jar is lighter, then you may have broken down some of the tannins.

If you really wanted a good test, then put the same number of minnows in both jars and confirm that the minnows in the bubbled jar survive at least as well as the ones in the other jar.

If the bubble test proved to break down the tannins, then starting aeration in your pond should have multiple benefits.

Otherwise, time and further inflows of water should reduce the tannin content of your water.

P.S. It is a good sign that your sunfish and minnows are apparently surviving. However, I would certainly feel more optimistic about adding expensive stocked fish after the water has cleared.

Good luck with your new 5-acre pond project!