The tannins causing the "tea stain" color of your pond are probably now mostly in the form of humic and fulvic acids. These are very weak acids, and are NOT generally a problem just by themselves. However, these are chemically reactive molecules, so your pond chemistry may be a little more complex than the typical pond discussed on Pond Boss.

If these acids persist throughout the entire year then your pond might even be buffered to be in a low pH (4.0-5.0?) range for most of the annual cycle. I definitely agree with the good advice above to do some water testing. However, it may be difficult/expensive to raise your pH to the optimal level for your fish if your pond has some significant buffering (humic/fulvic system) capacity. If my speculation is even close, then you may need to manage your pond as a "lower pH" pond. That is way above my pay grade, but we do have some experts to help if that proves to be the case after your water testing and some remediation efforts.

The good news is that the tannins are not DIRECTLY affecting your dissolved oxygen levels. However, with ice cover AND dark stained water, you may have very low photosynthesis for oxygen creation during the winter months. Some aeration may be of great value. Do you have any neighbor buddies with fish ponds and the same local conditions? Their advice may be much more valuable than some yahoo in Kansas!

Finally, I do hope your initial fish stockers are still alive. Don't give up on them until you have fished extensively during warmer water temps.