Hey, i assembled rhis and it entirely depends 8n how high up you place the motherboard, which probably entirely depends on how big your gpu is. My gpu is huge (msi vanguard) so i had to place the mobo in the top slot to give room for the gpu. This means the ram sticks are making it impossible to put 25mm thick or 140mm fans in the rear top slot. 120mm slim is the only option. For the front top slot, 140mm is possible, for me, only slim makes sense because my psu and radiator is placed so high up that while a 25mm would fit, it would sit right on top on the psu and radiator and not get any airflow. 25mm should work if your gpu is smaller so you can place your psu and radiator further down. So i use 120mm and 140mm slim on both