How many gph for manifold?

[Fishcrazy06]

So I am planning on building a manifold to run my carbon and go reactor and to also feed my calcium reactor. My main pump is pretty much at full capacity so I decided I would build a manifold and use a submersible pump to feed the manifold. What I am wondering is what gph pump should I be looking for? All plumbing and manifold will be made using 1/2" pipe. The two reactors are BRS reactors and the CaRx is a Geo 612.

Thanks in advance

 

[Reefing Madness]