If you tap water has reasonable amount of nitrate then do more water changes.
My tap waters got near enough zero nitrate in.
Ive just bought some aquamedic chemical that claims to get rid of BGA, seems to be a 4 day course.
If this doesnt work, then blackout it is.
I just cant seem to understand why its all kicked off all of a sudden, nothings majorly changed in months on my tank.