For starters, your tank is much too small for 5 goldfish. The 20-25 ltr tank you have equals about 5-6 gallons and that isn't large enough for one goldfish.

Buy a larger tank and let it cycle for a few weeks before adding the fish. Also buy a new filter system (the larger the better) since goldfish produce a lot of waste.

Are you scraping the sides of the glass tank each time you make a water change? Until you get a larger tank you need to do water changes at least twice a week (I would do 50% water changes) and keep the glass clean. Also buy a test kit and check the ammonia and nitrate levels of your tank. High ammonia essentially burns the fish and will lead to illness, disease and death.

There are many goldfish forums on the web so I suggest signing up for one, or more, and learn as much as you can about keeping goldfish. Check the link below for one forum..........

Here is a link that might be useful: Goldfish forum

Everything that Annz said, plus if your aquarium is in direct sunlight for a good part of the day, it is more conducive to algae growth (the rusty substance in your tank). I would move it out of direct sunlight. Good luck.