Water changes - 50% a week
Filter cleaning - at least once a month, every 2 weeks is better
Decor - if/when it needs it to remove algae, though stopping the algae growing is better.
50% water changes keep tank water conditions near to tap water conditions.
The brown goo that builds up on the filter media is uneaten fish food, fish poop, bit of deal plants when there are live pants. This all decomposes to ammonia which the bacteria turn into nitrate. The brown goo is sometimes referred to as a nitrate factory. Since we need to keep nitrate below 20 ppm, removing things that make nitrate helps to do this.
Decor grows biofilm on it and the filter bacteria grow in the biofilm. Scrubbing decor clean can remove the biofilm, and the bacteria. If the reason for cleaning decor is to remove algae, it is better to find out why the algae is growing. For example, with no live plants the usual reason is having the tank lights on too long.