o man, i remember when i was asking the same question not to long ago. well this is what i learned. a sump has many applications, it can be used to increase water volume, and be a place to place equipment. when you increase water volume, pH, salinity, and all that other jazz has a less chance of fluctuating and in a sense becomes easier to control. more water=better. protien skimmers can be placed in here along with heaters, calcium reactors, and anything else you can fit in there. as for the plumbing required to drain water into the sump, you dont need to drill holes in order to have a sump. you can buy a overflow box but many people have in the past recommended me not to, as they have been known to fail and cause a mess on the floor. if you want to play it safe drilling the tank will be your only option, but make sure the glass isnt tempered before drilling. i myself went ahead and got my tank drilled, you can do it yourself too if you think you can. you'll need a good drill, some sort of drill guide, a diamond tipped drill bit, and coolant to keep the glass cool. (water can be used)
as for a refugium, it can be a valuble addition to your tank. basically meaning a place free from preditation, small sessile inverts and such can flourish in here to provide a constant source of food for the display tank. it also can be a place for algae to grow that can be used to help buffer the drop in oxygen levels that usually happens at night when the lights go off by using reverse lighting patterns from your display tank. deep sand beds can be placed here too. a refugium from what i know can be a HOB type, sump type, or one place above the display tank's water to be gravity fed. recommended sizes for a refugium usually at minimum is 10-20% of the display tank's water volume. so in your case 20% would be 13 gallons. but many people shoot for higher volumes.
well this is what i know from the top of my head, im sure people out there can add much more. if you get tired of searching these forums for answers you should try books, there awesome. oh, and one more thing if you do decide to get a sump or refugium consider making one yourself, the prices of the commercial ones are ridiculous.
