The growbed is higher than the water level in the fish tank, and I have a hole in the side of the fish tank wall so that water from the grow bed can flow downhill through that hole and back into the fish tank, making a small waterfall.

I use the reflective material primarily to bounce light down into the water/gravel in front of it, which allows me to capture and hold more heat. It's for helping to keep the system's temp up as high as possible in the winters, but I leave it up in the summer because it's a hassle to take down and put back up. In the summer, the sun is high enough overhead that the light comes mostly straight down, so the reflectors don't do much reflecting in the summer. That let's me get away with leaving it in place without causing the system to overheat. But in the winters when the sun stays much lower, it catches and reflects the light.