Agreed. OP, you seem to have a lot of green spot algae on the glass, it can be quite stubborn to remove, but I find it comes off incredibly easy with a razor blade. I personally use Seachem's 46cm 'algae scraper' simply because I like the long handle, but any razor blade will work - it's completely fine to use on glass aquariums (not acrylic), and won't scratch provided you use even pressure flat against the glass, and don't get anything hard like sand caught on the blade. I used to just use melamine sponges (and they do work), but since switching to razor blades I'd never look back, the razors are just so effective. The scraper shears it off in one go.