I also had this trouble until I discovered after the second tap water test that we had nitrates over 200! So there I was merrily doing 30% water changes every other day to try to get under control and getting nowhere ( tank 180 litre) I then started to buy RO water from fish store and after a week of 30% every other day water changes we were at a safe level, and the algae reduced too! Good luck, hope you get it sorted soon, it's so frustrating when you just want to do right by the fish.