A healthy biological filter produces nitrates, not removed them, so adding more filtration will not solve the issue. Have you tested your source water for nitrates?
When you are feeding what foods & how often do you feed?
Each time you change 50% when your nitrate reading is 20+ you are only halving it at best, seems to me like you have a nitrate creep over time that has resulted in this experience. Increasing your water change frequency and/or % will correct this.
The other thing is fish respiration also produces waste that is converted to nitrate, so even when not feeding nitrates will naturally rise.
Unless you do a close to 100% water change at least once a week your nitrates never get to 0 and then slowly creeps back up again.