If we gave the 9004 DC the most conservative rating for heavy SPS and high fish load it is rated to 23.7 gallons. My opinion is the 9004DC is enough, I use the 9012 DC, my tank is set up now 4.5 months and is 215 gallons, I keep mostly montipora caps but all are small frags now. I have 5 anthias, a pyramid butterfly and 2 bangaiis and I feed 3 cubes of fish food per day. I also do vodka dosing and in all this time I have changed 15% of the water twice, last time was 2 weeks ago. My nitrate is undetectable, phosphate is .06ppm and I suspect most of this is when the fish were new I used an anti parasitic fishfood and the medication is a phosphate salt. I am using a little GFO now to drop this but I think the basic idea is the smaller skimmer takes less space and with other filtration methods and water changes it should be no problem. I will say that the 9012 fills the cup every 2 days since the Vodka dosing was started so one reason to go bigger could be the larger cup capacity, but the holiday cup 9004.145 is another way.