well to be honest there will be a slight constriction, the pipes them selves or the t pieces (depending on brand of fittings used) so this issue is possible.
You will find there will be positive pressure on the check valves cause of head height. the water will want to go the way of least resistance. I know this for a fact cause i have a similar situation but im "t"ing off a return pump from a sump to run fluidized bed filters. Just another thought on the fert dosing how will you know how much has been put in? you are running a pump into an unknown pressure or vacuum, how do you intend to measure amounts? One thing i can suggest is the GHL (profilux brand) dosing pumps and have their hoses coming over the top of the tank bracing.
Im not trying to shoot you down, these are very real concerns i have for your setup. I would personally try to keep it simple, that way there is less to go wrong.
The constriction you speak of isnt possible using class 9 or greater pressure pipe as the fittings are female, a male fitting will create a constriction. The only positive pressure on the check valve will be from the flow side of the valve when the fertilizer pump is activated. the negative pressure will be caused by the FX5 pumping water into the closed end of the check valve.
I will be using the EI method to fertilize, i will use a hopper to dry dose the fertilizer sump and the fertilizer pump will run for 20 seconds. That will be enough time to stir up the fertilizers and pump out the required dosing solution. Solution taken from Petalphile.com. Whatever fertilizer is left at the bottom of the tank will be minimal and diluted with the next introduction of fertilizer and water. The left over fertilizer will soon become a constant and i can adjust the amount of dry fertilizer to suit.
I appreciate the conversation and i dont think your trying to shoot me down.
As a hydro engineer and understand positive and negative atmospheric pressure pretty well so im certain that a Venturi will not occour.