[Gripenfelter]

What kind of skimmer do you have?

My old skimmer produced 1/2 cup of crap per week.

New one pulls 1 L of dark green matter every day. I found my nitrate count was higher if I turned it down to only pull out black crap.

Also I would do weekly 5% water changes instead of one big 20% one at the end of the month.

What food are you feeding the fish?

If your tank is fairly well stocked you might want to consider a refugium.