[Diana]

Is this the plastic hood?
Front half is a flip-up lid, middle half is the glass, and the narrow strip at the back has several sizes of punch-outs for equipment?

You might look for something like Rain-X to coat the glass with, but I have no idea if it is fish-safe. That might minimize the fogging.

Replacing the glass is very easy. If the old glass is glued in you will want to do this in a safe manner. Set the lid in a box with the glass down and see if you can slip a blade in where it is glued to break the seal. Otherwise, just start breaking out the glass. When the glass is gone, clean out the glue. Have new glass cut to fit. Match the thickness of the old glass. A few blobs of glue or silicone caulk will hold it in place, or do not use any glue. The glass will sit on the ledge even without glue. Remember when you take that lid off, though, that the glass is loose.

 

[Diana]

Hmmm
different definitions of foggy.
I was assuming condensation from the tank was beading up on the underside of the glass.
mustang suggests it is mineral build up.

If the glass is permanently fogged because of etching from minerals or the cleaners used, then replacing the glass or the whole lid is the only way to get rid of it.

Try the CLR. That cuts the minerals really well. Read and follow all the safety precautions.