You could try a Sambucus. They come in all varieties. There is a dark purple (nigra??) and a golden (southern gold) and some variegated ones as well. Some classify a Sambucus as a shrub. They can be a tree if pruned correctly, but they ca also be about 10' x 6' or more depending on the species. They grow fast .

Are you sure you want a tree hanging over your pond?

I have neighbor's mature Doug Firs and the Japanese Maple that I planted hanging over mine and it is a real pain in the rear to clean out all the debris. I'm sorry we built the pond where we did. The conifer needles are the worst. They clog the vacuum too.

If you are looking to reduce algae by shading the water using aquatic plants would be less work.

I use a large styrofoam floating island planter with evergreen plants in it to give my fish a hiding place in winter when many plants are dormant.

It might be easier to make suggestions if we knew where you were located and how precipitation you get. Temps in winter? Etc..

oops, sorry 1st post-thought it would list my area info when I posted. I live in Lacey, WA-Average rain fall is less than 2" in summer, 6-8 in winter, fall and spring 2-6. Temps are average, usually dont get snow, 40s to high 60s in spring and fall, 70s to mid 90s in summer. Hope that helps. Also, I really do need a tree to give the pond some shade, but it will be back a little bit from the pond, and im not worried about leaves getting into it. thanks for everyones suggestion so far