The normal knee-jerk reaction to "tea-brown color" is tannins of course, and that may be the case here. But I have often found people's descriptions of color to be problematic and often leads the owner to try and fix the wrong thing.

The standard knee-jerk fix in forums activated charcoal...unfortunately. Activated charcoal can indeed clear tannins if say a few thousand dollars worth was used.

If the water really does look like tea and you suspect tannins the trick is to find the source and deal with that. The source can be leaves (just like tea) and bark which can be in mulch or flaking off a tree. Trying to clear tannins without dealing with the source means the water will never clear.

The most effective and cheapest way to remove tannins (after the course has been handled) is by water changes. Oxidizers can also be used but way more expensive.

However, I dislike knee-jerk responses. There are some things in the post that don't add up. The two ponds are connected but only the water in the upper pond is brown. That's impossible. The only thing that could explain this is the water in the upper pond only appears to be brown. This can be due to the color of the pond bottom, dead string algae (any macro algae), or maybe reflections from a building, trees, etc.

So I suggest first confirming the water is actually brown. Like filling a white 5 gal bucket with pond water to check the color. Or sinking a white object (white 5 gal bucket) into the pond like a Secchi Disk. Do it in both ponds. Near high noon is the best time.

Couple of side notes...

Plants only provide oxygen when the sun is out. Without sun plants consume oxygen. When dealing with fish we only care about the lowest level of O2 and therefore plants are a negative O2 wise. Plus they add organic matter which also consume O2. That plants add O2 to ponds is one of the many and often repeated pond myths.

Posting pH as "high" can lead to all kinds of problems with wild advice. Much better, and about the same number of key strokes, to actually type the value.

Algae would not reduce tannins or any of the other causes of "brown" water that I can think of.

If by "high" pH you mean 16, 17 then yes, that might interfere with algae growth. If by "high" you mean 10-14 then no, algae would be just fine.

Fish don't have a problem with tannins or most "brown" colors. Tannins can reduce O2 very slightly but fish always like added cover so on the whole they probably like it.

Cleaning the filters wouldn't have a huge impact on tannins. Tannins are just really small bits of organic matter so it decays in the water and eventually disappears. Way too small for pond filters. If more tannins are being produced then the water never clears. In theory if the pond's ability to decompose waste were increased the tannins could decay faster. But that's theory, in practice it's very unlikely to matter imo. For there to be enough tannins in water to appear brown means there's a lot of tannins and so even if you removed all the plants, cleaned the pond to spotless and added lots of air pumps, the water would likely still remain brown.

thanks for the feedback. A little clarification. The PH is about 9.0. I added baking soda last night, it did reduce the PH to around the 8.0 mark.

There are no active leaves in the pond. No bark or mulch residue. We do have leaves in the fall, but I get them out as soon as possible.

I do use a product that removes sludge from the bottom of the pond. It appears to have worked well. It is called Natural Pond cleaner, which puts back good bacteria when applied.

We had this same brown color last year, for a month, but when the green algae appeared on the rocks edges, the brown water went completely away. That is what I need.

Is there such as thing as brown algae? I do not see any residual on the rocks. I could do a water change again, maybe 20% but I am not sure if that would help.

Like I said, the fish appear to be very happy, as well as the several frogs.

I think there is algae in a number of different colors.

Other than the testing for actual color suggested above, I would wonder if you put in any new plants or re-potted some. Bagged potting soil is notorious for lots of peat moss. It is great to improve some soils but does leach tannin.

Reduce that PH slowly to avoid shock.

In most cases bacteria off the shelf is just a feel good measure that doesn't really do much besides making your wallet lighter. Filtration and aeration is usually all that is necessary.Regular tests for water quality with a good quality liquid chemical test kit saves a lot of time and effort.

I do use a product that removes sludge from the bottom of the pond. It appears to have worked well. It is called Natural Pond cleaner, which puts back good bacteria when applied.
The sellers of these products depend on users seeing an improvement. But of course there's no actual way to tell by looking. If the pond is doing well owners attribute the goodness to whatever they did, like add a bottle of something. Human nature and a pretty good selling scheme.

It is possible the product added the tannins, but I doubt it would color the entire pond...or the upper pool only...or whatever is going on.

We had this same brown color last year, for a month, but when the green algae appeared on the rocks edges, the brown water went completely away. That is what I need.
All things are possible...but since we have no idea what this brown color is there certainly isn't any way to draw the conclusion that some algae removed the brown. There are a lot of scenarios that could explain it.

Is there such as thing as brown algae? I do not see any residual on the rocks.
There are brown algae species but none that I would call tea brown. The brown kinds are generally macro type, like plants.

I could do a water change again, maybe 20% but I am not sure if that would help.
See, now to me that's really interesting. You call this tea brown...but it only appears in the upper pool. No comment on doing the color test, no explanation on how two connected pools can have different colored water. But you doubt a water change would effect water color. That leads me to think you know something you're not wanting to comment on. But you would like help on fixing the problem. Fascinating.

thanks for the feedback. A little clarification. The PH is about 9.0. I added baking soda last night, it did reduce the PH to around the 8.0 mark.
Baking soda in water has a pH of around 9 to 8.3. Adding baking soda would not reduce pH from 9 to 8. Other things, like GH is what interacts with baking soda to get a 8.3 pH. But most people don't mess with GH since the chemicals are scary sounding and 9 pH is fine.

What likely happened is the pH in each test just happen to be different. Different time of day, different depth, different location, difficult to tell the difference in colors, etc. pH tests are very hard to get right and confuse a lot of people into doing harmful things to the pond. Adding baking soda isn't normally harmful unless ammonia is present but I'm sure you tested ammonia first and found it to be 0.

I suggest to people who don't really want to know too much about pH that it would be way better for the pond and fish if they threw out their pH test kit and just use a KH test. KH is how you determine how much baking soda to use. pH test result just cause way more harm than good. Unless of course a person is studying how pH works.

I assume the 9 pH is what was called "high" in the first post. It's up to each pond keeper to determine what pH they want. I for example wouldn't call 9 high in a Water Garden or Koi Pond. Many pretty serious fish keepers allow pH to be around 9. Not perfect, but fine.

At any rate I wish you good luck with whatever is going on.

You say you had the same tea color last year; was it the same
time of year? We have a mulberry tree, near our pond, every April
the tannins and berries from this tree turn our pond tea colored for
about two weeks. It happens every year at the same time.
We find cleaning the filters more frequently helps clear the water
and we also run the skimmer in early Spring to catch the berries.

Look around, maybe something that blooms in your garden is getting
into your pond and turning your water tea colored.