the male betta in my friends 18 liter tank is bullying their female ... BADLY ... is there anything they can do to prevent this or do the pair have to be separated into different tanks, i know aggression is bad in bettas but this male is extremely bad

Hey so I have just set up a new tank 28L (~7g) and am waiting for the food to decompose and start the cycling. My tank is 31cm long, 30cm deep and 30 cm tall. I currently have a decent amount of hair grass, blyxa j. and java moss and I am planning on putting in another plant in the hole on the...