Since you haven't gotten a reply, I'm just going to throw some ideas out there.

Sometimes algae bloom is caused by lack of oxygen in the water. Since you said you have no plants, that's a possibility if your fountain or waterfall isn't doing enough.

You said it's mostly in the shade. Do leaves fall in the pond? Could there be too much nutrient left in the pond after your spring cleanout? Could fertilizer runoff be getting into the pond?

Do a search for types of pond algae. Maybe the kind you have doesn't respond to the Tetra Pond you are using.

I'm not a big believer in adding stuff to the pond, but a lot of people put barley straw in the pond to clear it. But I think aeration is worth looking into.

Good luck with it. Wish I knew what would help.

Thanks so much. Wondering about the oxygen issue and think I might add plants. I only use organic products in the garden, but even that could cause a problem if 'run off' is an issue.

I use a little solar powered aerator I got from a big box store. It keeps about 200 gallons of water clear but I still have the good algae on surfaces of rocks. I would not use this aerator for total support of fish however because, um, it doesn't work at night. :)