What are your current water parameters?
This sounds like an issue with nitrates and phosphates, along with low flow, as Cyanobacteria loves nutrients and low flow.
There are two articles on my signature that discuss algae battles, but you simple aren't taking out enough nutrients as you are putting more in.
It is recommended to do 10% water changes weekly to combat growing nutrient issues. So if nitrates were 50, it would go down by 5...
You will need to make a long term battle plan to solve your issue. Increase flow, cut back on feeding minimally every 3 days, do large water changes every so many days and manually remove as much cyano
and algae that you can in the process.
Remember, this won't be solved over night. It could take easily over a month to combat what is currently established.
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