Sump Question-Rubble Rock

Like others have said I would ditch the LR rubble since eventually it is just going to become a nitrate factory as it fills up with detritus because of low flow through it as well as the fact that your fuge drains basically directly onto the rock rubble

Also if you dont have a way to pick up the flow through the fuge I would add a small power head in there to get the water moving around, there is absolutely no reason that water needs to go slow through the fuge
It's not like a skimmer where "contact time" matters, the macro is going to feed off of nutrients in the tank and when the macro starts to grow you dont want a bunch of dead zones where there is no water movement to cause the macro to die off and release nutrients that they have absorbed back into the tank

Basically take out the rubble and add a powerhead to get some water moving inside that fuge

 

That's actually a good idea. I may try that.

Another thought is to get away from the total flow concept, and look at what is really going through the fuge or sump. On the extreme side in my system, assuming an 800 gpf flow, that would be a lot for the first 20 gallon tank just for a skimmer. The second 20 gallon tank with macro, it seems about right, could even be a little more. The third tank, a 40 breeder, the flow is not very noticeable. In the last 100 gallon section, the flow is barely noticeable.

Under the tank systems are very confined, so it's easier to apply generalized rules. Basement and fish room sumps are more wide open to possibilities.

Hope any of this is helping the OP.