Im getting a 20L fancy goldfish aquarium for xmas, and i just have a few questions.

-Should i cycle the tank for a week or so before putting the fish in?

-When i do the monthly cleaning, do i clean the gravel and all the decor and plants, do a 100% water change? Or do a 60-80% water change, clean all the stuff ext..?

-Do i change the power filter bag every 3 months?

-Would 4 fancy goldfish be to many for a 20L tank?

-Do i need a heater for the tank?

-Do i need any other water additives other than aquasafe?

if you have any other reccomendations or advice let me know