Cleaning fish tanks is far from fun. It is especially difficult to remove the gunk from the corners.
To help with this problem, use a little hydrogen peroxide on your cleaning brush and you can easily scrub the aquarium glass and corners, getting rid of the scum that forms inside of the tank. I use a cheap generic brand and it works just as well as the name brands.
Just make sure you rinse out the tank *thoroughly* before adding your fish into it.