The intake is behind the BG.
I drilled 5mm holes between the rocks so you can hardly see them. The heater is behind there too and I have a small nano circulation pump / powerhead bhind the BG so there are no dead spots, also so the heater is more effective.
Unfortunately, I have 2 Pseudotropheus Acei in the tank and one of them is now living behind the BG and unless I remove it again, It is staying there until I do a big maintenance job!