A 21G fuge for a 180G would probably not give you what you're looking for. You need to consider something more along the lines of 50G to provide that kind of nutrient export.
Have you looked at all the other potential causes of your HA problem?
In my 180G, I have a refugium for protecting pod population in a 5 gallon bucket and a fuge area in the center of my 55G sump with cheato for nutrient export. the fuge area in the sump is about 25 gallons. I also have the bucket fed by my phosphate reactor. I guess both work as nutrient export because there is about 3 inches of sand and a ball of cheato in the bucket as well..
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