That is incredibly weird. Glass is extremely inert. There is almost no way the glass itself can suddenly become 'cloudy'. You're certain it is the outside surface of the glass? If so, their are only two potential causes that I know of:
1. Something is creating a film on the outer surface. Is the tank near a kitchen and have you fried and foods lately? Have any chemical cleaner been used near the tank? Has anything been done recently in the house that might have caused a fine smoky or oil residue to get into the air (for example,you'd be surprised how much oil vapor can get into the air from the simple act of frying french fries)?
The problem with this potential cause though is that only the front panel is affected and not the sides, and usually a glass cleaner like windex or vinegar will at least smear it ... if not completely remove it ... letting you know it is indeed a film.
2. Is the glass tempered? Improper tempering can cause glass to look hazy. AFAIK though, when it does occur, it is immediately noticeable from the moment the manufacturing process is complete. So, the haze would've been present from day 1 rather than suddenly occurring out of the blue.
That said, if cleaning doesn't improve it in any way, the glass IS tempered, and it is only occurring to that one panel of glass, I would investigate the possibility of defect caused by the tempering process.
If the glass ISN'T tempered, than it has to be something that is building up on either the inner or outer surface.
