If it's really bad, you probably want to remove your fish to a pitcher, drain the tank, and clean it completely. Get your pet store to recommend a soap.
The new skimming filters do a great job. They constantly removc the scum from the fish tank. You can use the plastic stick with a blade to remove the algae and scum from the front of the tank. Remove the rocks and other aquascaping decor to scrub with a brush.
To clean a aquarium a special type of fish is used. This fish cleans the aquarium neatly. The scum is removed. Cleaning weekly twice is also preferable. Too much of fish and also decorative items must not be dumped in the aquarium.