Your parameters seem pretty good. Still only the Sailfin Blenny in there? Do you feed heavy or only enough for the blenny? You could be right, maybe the system is starved for nutrients.
You've got a pretty small tank with a big skimmer (BM5 rated up to 140G) and a refugium, and the system is just being stripped of all the nutrients. 6 month old tank with no algae and the refugium dying off are pretty good indicators.
At this point it's almost like you're running an ULNS (Ultra Low Nutrient System). I don't have a lot of experience with these systems, but I know that the theory is to almost starve the system and then feed it only the nutrients you want. The Zeovit method is an ULNS and a good example.
You could go a couple of ways at this point to increase your nutrient levels:
-Add some fish to the system and start feeding heavier
-Start feeding the corals directly using Reef-Roids, Reef Snow, Red Sea Reef Energy, Aquavitro Reef Fuel Etc. Etc.
-Take the skimmer offline?
I'm not an expert and personally have never had an ULNS or a problem similar to this, so hopefully some who has direct experience can also chime in.
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