My plants are growing vertically fast but not sideways. How do I make them
grow more bushy rather than having them stretching their stems to grow
vertically.

My tank is 3 x 2 x 2 ft.
I turn on my CO2 supply, and 4 tubes of 40W florescent tubes for 7-8 hrs
daily, and apply liquid fertiliser weekly.

Really puzzles me, could anyone advice me, please? Thanks!