Howdy

welcome to 3reef. to answer some of your questions, live rock will be your primary source of natural filtration. live rock plays a huge part in converting nitrites into nitrates and breaking down ammonia, it's a natural process that you should research to gain a better understanding. as far as your water volume is concerned....is the tank you speak of 100 gallons? if so, you will have water volume displacement due to the amount of live rock you decide to place into the tank. imho a good rule of thumb is 1.5 to 2 pounds of live rock per gallon but it depends upon the live rock you're using and you'll find a good balance once you've been in the hobby (or lifestyle) for awhile. you can always take into account the lr (live rock) you'll place into the refugium and the water that the refugium will hold as a final number of lr vs. saltwater. hth.