What should I use to clean whitish residue from reservoir?

[DarkenReaper57]

Hiya, I recently purchased a plexi bay res from someone in a package deal. At the top of the res, there is some white residue which I cannot seem to clean off. I think it might be from water wetter, but I am not sure. I tried using strong soap and let it soak overnight...no luck.

I'd try vinegar but im not sure whther or not that would react with plexi in some unfortunate way. The res might be made of an acrylic, honestly im not sure.

Any ideas? I'd scrub it off, but I cant exactly break open the res .

Thanks.