how to get rid of spirogyra in a shrimp tank

manual removal, more water changes and keep the filters cleaner. that's all i got for you vise attempting to starve it by cutting back on your dosing.
 
with the abundance of food would the amanos still pick on the algae? or would they also compete with food with my PFRS?

Amanos are, simply put, machines. If it's edible, they're gonna munch it. :) They are constantly grazing.
 
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