When you do water changes you need to work with the total water volume so your 10% changes, from what you have described would be 10 gallons. A sump is just extra water and a place to put other filtration, IE. a skimmer or reactors. Arefugium is a section to grow macro algae and some people use them for a Deep Sand Bed (dsb) if they do not have one in the display. If you do not have a protien skimmer it is highy recomended that 50% per week water changes be done. This will remove the waste that would other wise be removed by skimming. I feel that a good skimmer is one of the most important parts of any system. I highly recomend Coralvue's Octopus skimmers they (IMO) are the best for the money. For 100g total water volume you should look at a skimmer rated for around 150g.