Basic premise of a sump is water flows out of the tank to the sump via gravity(usually through the bottom or back) and is returned back up to the tank with some sort of pump either in line or submersible.....the sump is your filter. Most folks use some sort of sock or mechanical filtration first, then water flows through and around your biofiltration media(matrix, ceramic rings, etc.) and returns back up to the tank with the pump. Some people use seperate chambers in their sumps for various things, but its probably mostly unneccessary. As long as the water comes into contact with your biomedia, youre good to go. In my opinion, the simpler the setup the better. Most sumps have drains on one end, pump on the other with bio in the middle
Its actually a very simple for enormously effective way to approach large tank filtration. You can put anything you would normally put in your tank(heaters, dosing equiptment, etc) into your sump, including fish if need be. It also expands the total volume of your system which is a huge plus......more water means a healthier system
im in the design process of my 300 gallon setup and ill be using a 75 gallon sump with a pump rated at about 2700gph@ 6'. Most people recommend a sump equivalent to 20-25% of your total tank volume. So 2 55's could work, although I dont know much about using multiple sumps.
Drain style will depend alot on where you decide to put your overflows and available space around your tank. Most reefers use either a Beananimal or Herbie style of overflow which serves two main purposes 1) some sort of emergency drain and 2) a way to silence your drains resulting in a super quiet system.
Lots and lots of options with regards to return pumps....
