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Or muriatic acidWelcome to Reef2Reef!
I've seen this kind of thing before, and if it's what I think it is, it's actually that the glass has become etched. I don't believe a typical cleaning product will do it, I think you might have to try to polish it out. I will say that it should look a lot less noticeable when the tank is filled.
Good luck!
Sometimes glass gets etched in a way that an acid can't get offOr muriatic acid

The glass has been etched and it wont clean off - can only be polished out with a polishing compound.
I guess it depends what that is. It does not look like dried out coraline algaeSometimes glass gets etched in a way that an acid can't get off![]()