Find out if it's actually tail heavy.

There are a couple of tests that show you whether or not a model is tail heavy. It beats guessing. BTW, tail heavy models usually have increased elevator effectiveness. And if a model is overly tail heavy, you'd have your hands full flying it. The pitch sensitivity would be excessive.

From what little you've described, it sounds like you might have a wing incidence problem. That's often the case with biplanes. Do you have any way to adjust the incidence of the top wing? The top wings are often the problem.