The only items that appear anomalous to me are the Media Reactors. It appears that they receive their water prior to Skimmer treatment and return it after skimmer treatment. I wonder if they might not better somewhere else in the loop. I am just not sure and await a more informed response from another.
At 550g total, one has either a lot of plumbing or a missing component. I don't see anything glaringly absent from the diagram. Since this is a one-story install, I can't foresee that much plumbing. My numbers point to a system volume of roughly 465g. 280g (Main) + 80g (add'l three displays) + 52g (two 'fuges) + 7g (skimmer) + 45g (sump) = 465g. Where I am going with this is that this could be tweaked a bit in terms of pumps and redundancy? A larger system pump might allow you to tweak flow out of the sump to the four displays via a manifold. This would seem more efficient in terms of both maintenance and electrical consumption.
I wonder, too, if a way might be found to feed the Ca Reactor without that additional pump.
Just my thoughts, and I await more accomplished system designers to chime in...
It looks great, if my comments don't indicate that.