Offgrid living

My rainwater collection begins with a first-flush and debris removal tank, but it is not ideal; all the water flows through it and in a big rain it can stay stirred up. So it has debris and because it is necessarily open to the roof this means some bugs in there. The outlet to the storage tank is wrapped with three layers of window-screen cloth, but the overflow is open to the drain. Frogs (cute little coqui tree-frogs) come up the drain and inhabit the tank. The storage tank overflow ties into the first flush overflow, so once in a while there is a tree frog in the first storage tank. There are no bugs in that tank so the frog will die if I fail to capture it (which they are very wily).

How can I keep a frog from climbing up the pipe while being sure I do not obstruct water going down the pipe?