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First of all, I'd like to say that I've never had an outdoor pond, nor has my brother who is building one.

Could anyone tell me why the water turns green when it rains and if that is normal? What could he do to stop that?

Also, he wants to use a pool filter for his pond. Basically, it's a pump with a strainer in it for mechanical filtration and it passes through a large tank of sand. The sand, however, would be replaced by activated aquarium carbon and bio rings that would be replaced every year (this thing could probably hold 20 pounds of media). Would this be sufficient filtration? I don't see why not, since it's just a pump with a media basket. It's a little unconventional, but he already had the filter for a pool that he never bought (lol).

Thanks.

He will need to add a UV sterilizer.Most ponds become green without one.

The water turns green only when it rains? If so, perhaps there is some kind of chemical in the rain (acid rain?) Does it stay green?

I know of some people who used a sand filter in their (now disbanded) koi pond. It sounds like it would do the job, but I'm wondering if or how much that biomedia would affect the flow...

With sand in it, the flow of the filter is fine, but it looses it's prime since it is not below the pond. He is hoping to add carbon to the filter and put the pump below the ground to keep it primed.

It seems as if it stays green after it rains, even when the turtle isn't in it.

To me it sounds like something in the rain, if that's what brings on the color. Is there any algae in it? Perhaps the color is intensified after the rain.

There doesn't appear to be any algae. You can dump the pond out and it's spotless. I'm hoping that after the filter stays running with new media, it stays clear, but I do think it has something to do with the rain.

Marisa you are correct its the rain! Your rain is high in nitrates and nutrients. Good for everything but ponds.

When it rains it adds nitrate and nutrient rich water into your pond. The nutrient levels spike, the planktonic algae in your pond has a sudden huge nutrient supply and so it takes off turning the water green.
Even though you cant see the algea its there in small amounts and then it grows like crazy cause the rain feeds it.

I would suggest getting a uv clarifier like mentioned to help fight your problem. Normal a pond would just turn somwhat cloudy.

After a rain my pond turns incredibly clean,lol. I use a few teaspoons of pond magic and the next day its crystal clear, you just see the algea on the bottom, but mine is clearly from the sun. I dont have a uv clarifier but it does sound like a good investment

its actually called pond care/algeafix sorry