I don't have it all in yet, the pump arrives Friday, but I clearly just spent too much money trying to do something similar. But getting the timer at Radio Shack, and the peristalic pump and a custom T from http://www.automatedaquariums.com
was definately past $100. I feel a bit stupid now.
Can you explain the back pressure concerns on your canister?
And not to harp on a recent subject, but are you planning on putting the T pre, or post canister? And I haven't even stopped to consider questions like how this relatively high concentration of cold fert will affect an in-line heater or CO2 reactor. Gosh, it's all connected.
I can't wait to hear how it turns out - 'cuz I'm trying to figure it out as I go...