i had something similar, but i didn't have to go a whole floor down. mine was a 195 in wall display, and what ended up being 3 coral tanks all plumbed into a common 125 gallon tank which was set up as a sump with refugium.
here's an early shot with a 26 gallon acrylic tank suspended from the ceiling (i eventually replaced that with a 30 breeder)
here is the 125 sump. i think the refugium is about 50 gallons or so.
then i removed 2 of the breeder tanks and added a 150 gallon shallow coral tank-
please excuse the wire mess...it was a work in process, and quite functional.
i used a mag24 for the return on the 195, and assorted pumps for the returns on the other tanks. quiet one 4000 was used for that shallow 150 and smaller mag drives for the breeder tanks.
i always wanted to build a pipe system with valves so i could redirect the water whenever i wanted, and use a single pump for all. i found the multiple pumps to be a big P.I.T.A.
btw, i would never have used HOB
overflows for this setup, and i suggest you drill those tanks also. if something happens, it's going to happen big. say one of those HOb
overflow boxes loses siphon. that whole sump will be on the floor of the room behind it.