HI everyone, I was wondering what an undergravel filter does exactly and what I need to do to make it run smoothly? I got my hands on one for free from a friend and have set it up in my 20 gallon tank, but does it need any filter media? how does it work? i’ve got it in the tank & attached the air pump to the tube things and that’s it. sorry if this is ignorant lol but I have no clue how I should manage it at all.