No pond experience, but I do prune my mother's magnolia tree.
I do not believe there is any "risk" for sinking magnolia tree tops. The wood I cut seems pretty soft to me, so I expect your treetops will degrade over time. (However, that may be just fine, because your habitat is constantly evolving. You can put in some more tops every other year to keep a range of habitat density.)
The leaves I see on magnolias around here (and farther south of me) are typically very large. I would NOT want the leaves in my pond.
If they don't readily fall off, you might be able to burn them off. (If you have a very safe place to perform a burn!) Pile the treetops up, light the dried leaves on fire, hit the pile with your leaf blower so the entire pile is quickly on fire, spray your hose on everything after 2-3 minutes or as soon as the leaves are gone.
Sometimes fire hardening makes softer woods last longer. Burning off the leaves might actually make some of your smaller branches last longer in the pond.
Good luck on your project to make your fish happier!
