Roll / fleece filters are just mechanical filters, like skimmers, so everything about skimmers applies to roll filters too, except skimmers oxygenate, whereas roll filters don't.
The easiest way to compare scrubbers to rollers or skimmers is to think about food. Food is "organic" and should not to be confused with human food "organic non-gmo food etc" that you eat.
Nutrients, However are "inorganic", and should not be confused with human food nutrients that you eat.
Scrubbers remove inorganics from the water: ammonia, nitrite, nitrate, phosphate, CO2, copper, etc, but do not remove food particles.
Scrubbers add organics to the water: Glucose, amino acids, vitamin C, etc. Oxygen and copepods too.
Protein skimmers, however, do the opposite. That's why they have the word "protein" in the name.
Skimmers remove food particles and copepods from the water, but do not remove ammonia, nitrite, nitrate, phosphate, CO2, copper, etc.
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And also...
Zero Nutrients
Many people newer to aquariums and especially scrubbers have heard that reducing nutrients to "zero" (nitrate, phosphate, etc as measured by hobby test kits) is not good for coral growth. The answer is, it depends. "Zero" is not really zero of course; hobby test kits don't need to test super low levels like lab tests do. In other words, there is always a level of nutrients there, even if you have "all zeros".
First thing to know is that natural reefs always test "zero" for nutrients with hobby test kits. Yet, they have very high growth rates; acro's can grow two feet in one year. But yes, many hobbyists do see less growth with zero nutrients. Why is this, and what is different with tanks compared to natural reefs?
One answer is food particles. The enormous amount of visible food particles (and invisible dissolved particles) in natural reefs is so high that at night some divers can't see their own hands. And the enormous level of photosynthesis on reefs consumes most of the resulting nutrients. But hobbyists can't have this level of food particles because of nutrient build up, so they reduce feeding, to as little as 1/1000 of natural reef levels. Thus, corals starve.
So one bandaid to help with this is to keep tank nutrients above "zero". This feeds bacteria around the corals, and the bacteria then become coral food. Softies take it in directly, by inflating with water, and polyps eat the microbes and pods that eat the bacteria.
This works but is self-limiting, because in most cases higher levels of phosphate is going to slow the coral growth, and also because bacteria are hard for polyps to grab. So the trick is to keep food particle levels up high, and keep nutrients down low, at the same time. In other words, try to duplicate natural reefs.
This is where algal filtration comes in. Just like on natural reefs, algae remove nitrate, phosphate, nitrite, ammonia, ammonium, etc, but do not remove food particles. In fact, algae will add food to the water, by putting glucose, amino acids, vitamin C and other sugars into the water just like some people dose. And just like carbon dosing, the glucose feeds more bacteria which both feeds the corals and consumes more nutrients.
And don't forget the last step of feeding some or all of the algae to the fish. This gives fish the type of fresh living algae that they want, without adding any new nutrients to the water from outside food sources.
