Overflow question

[Rikko]

I can't quite visualize what you mean there.. Can you describe the system with a little more anal detail?
Regardless - a strainer always increases the chances of failure, as it takes smaller material to plug the drain and cause the return pump to start flooding the tank. Is it likely? Not very. Unless you add lots and lots of big sheets of nori, there shouldn't be all that much that's going to plug up large section of the strainer. I would proceed - many systems use a strainer without problems.