You're more likely to have pressure differences from the front to the back of the line if you set the tubing in a spiral. A grid or manifold provides the same filtration effect using less tubing with even pressure across the system. 1/2" will work fine in this type of setup, with only one pump needed. I had a smaller pump running a similar system in a 20L. Wash the heck out of whatever substrate you use. I used turface and even with thorough washing the system blew out fine dust into the water as the substrate degraded from the constant flow of water.
Easing my toes in the water