Tank Background

[JasonandJill]

This has been bugging me for ages, and as i'm off work due to the atrocious 1 inch dusting of snow in Bristol, I'm going to try and sort out the background on my tanks.
The problem i've got is large dark areas and smaller dark "spots" on the background when viewing my tank.
As I can't get at the back of either tank from the sides, not a great deal of room reaching down behind, and the fact i've got to disconnect the hoses for the external filter, I'd like a bit of advice beforehand.
a/ Is it caused by the backing not touching the glass
b/ Is it because water has got between the backing and glass
c/ Something else
Any pointers to a pretty dumb question appreciated!!

 

[moochy13]

almost always condensation i find. press the backing against the glass and see if it changes how it looks.

 

[VaegaVic]

d) All of the above.

 

[JasonandJill]

Thanks for the replies, I'll just pull it off and with enough swearing and cursing I should be able to do all of the above. Maybe even get it back on again without swimming with the fish