A filter would add surface area to where my benaficial bacteria could grow and take away particles of food or waste. But with a small bioload, my 130+ pounds of rock has enough bacteria to handle my bioload. The ocean doesnt run mechanical filtration, it gets new water constantly running through it so I do 3-4 small water changes a week taking away unwanted chemicals in my water plus 5-7 gallons of fresh top off water a week . I have a perfect CUC to where I never see feces or the like on any surface. I understand what mandarins need to eat and trust me, I do not NEED a fuge. I monitor my pod population closely and he has not even put the slightest dent in it. Both my mandarins waist and my pod population seem to be growing. I know a sump would add volume to my tank, to be honest, I have one...with a skimmer. But my intake and return pump won't run at the same time so I've gone without it. Also I have 7 fish in a 90 gallon tank so I'm fine with my volume. I think the only fish I worry about starvation with is my cleaner wrasse. The little guy has such a high metabolic rate, I'm forced to feed my tank shrimp every day.