Yeah I was attempting to point out that equation isn't useful here, it's only for sizing pipe when you have a desired flow rate and velocity or solving for a different variable with the other two known. The velocity will actually depend on a few things including pressure difference, density (which is variable as well due to an inconsistent mixture of air and water), major loses from pipe friction and minor loses from pipe fittings. While it's completely possible to calculate this all it would be a complete waste of time especially since RC has been nice enough to make a simple calculator that estimates the required pipe size based on flow rate.
http://reefcentral.com/index.php/dra...flow-size-calc