20 cm pots? sounds kinda small, i use 2.5L minimum, are u planning on vegging in small pots and transplanting to larger?? sorry not being rude just confused with your question
U can google any weed question u have and if there is a thread here google will provide u with a link.
I suggest reading the newbie section, i did and alot of your questions will be answered correctly , not just peoples opinions and ideas.