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 3% food-grade 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 get the pictured hydrogen peroxide from Amazon in a 32oz bottle for about $14.00. While it cost more than the stuff you can buy in the brown bottle, it is food grade and known as being non-toxic to people, food, pets or plants.
Which ever type you choose to use, don’t apply it into the tank with the fist present and, just to be safe, be sure to rinse out the tank thoroughly before adding your fish into it.