What is this red stuff in my stock tank pond?

6 months ago
last modified: 5 months ago

I have a stock tank pond that I've had water in for 3 years, and goldfish and snails in this year. Each year, it gets this red sediment within 2 days of filling it. As you can see in the photos (or maybe not), the red is NOT on the surface of the water or even throughout the water- it settles on objects under the water. The bottom is covered, the cinder blocks I use for plants are covered, even the little plant leaves beneath the surface are covered.

My husband thinks it's algae, but I can't find anything online that looks like this. As you can see in the silhouette on the surface, the stock tank is relatively near/under 2 huge spruce trees. The male cones are red, and drop in the spring. But I would think that wouldn't continue to make the pond red the entire summer (but I really have no idea).

Other details: I live in a high desert valley in south-central Colorado, USDA zone 4. Our soil (and water) is alkaline. This pond was filled with town water, and I added the fish 3 weeks later.

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