Clematis armandii is a large ish leaved evergreen. But then all evergreens drop leaves throughout the year. The only way leaves will affect the fish is if they are left to sink in the water and rot. They normally lose the most around spring time- replacing old evergreen leaves with new.
Possible ivy, but it depends on the support you have for it and the eventual weight and sprangling the support can take.

I would assume that a poisonous leaved plant would affect the fish?
Any other ideas?