Sorry for all the questions, but MOST every tank related issue can be resolved through a process of elimination.

Forgot to ask, are you running a skimmer? Yep, even though the systems are linked and you're only having algae issues in one, I want to know if you are running a skimmer?

I think I know what your problem is, but instead of typing a book, I want to try and find something I read on line a year ago which says the exact same thing. Back soon.