Kathy:
For a fuge using algae to be successful at removing nutrients to a noticeable level and effecting low pH at night, it needs to be:
1)At a bare minimum, 1/3 the size of the main a display with a very modest bioload.
2)Good enough flow that detritus cannot settle between macros and on the bottom but low enough flow to allow algae to anchor(culerpa) or maintain it's form and not be released into pumps (cheato) and blow sand/mud all over the place.
3)Pruned regularly and completely removed from the system. (feeding it to your tangs puts the crap right back in your tank)
4)Additional supplements added for the algae to grow (cheato need iron)
5)Exceptional skimming.
Successful tanks that I have seen using refugiums are rare. I have only seen 2 in person. Both were over 3/4 the size of the main display and both used far more than the recommended rating for skimming. 1 had a bioload so low that I swear this guy was looking to never, ever change water. The other had a much, much heavier bioload, but used much more skimming.
I have never seen a tank that 90% or Manhattan Reefers would consider a "good" tank using those miracle mud, HOB systems. Never.
Mike (slamajama) is doing his right for the room he has available. He's not trying to control nutrients. He's not trying to balance pH at night (overrated problem which a good skimmer can correct to acceptable levels). He wants a better pod population for his wrasses. He's going with a section of his sump with no substrate. Just some LR and high flow. Ambient light from his main display will be a sufficient food source for algae. No cheato or culerpa. Film algae is fine. The stuff we scrape off the front glass.
It's not what I believe. I don't make this stuff up. I'm not smart enough. It is what I have seen. 1/3 is far too generous.