If its a 150g. then the dimensions are not 6' x 2' x 2'. Im telling you this so that you don't over or under stock or tank. Water volume is important for rays.
As for my other input...... there was a thread where a guy had a tank which he called a Stingray River i believe.. could be wrong but it was something cool like that. It had a DIY pvc rack inside the tank where he had plants and soil about the rays. Gave it a really amazon rainforest type feel, very colorful and aesthetically pleasing.
you might be able to pull off a mini version of this design. it would also assist with keeping your water parameters in check.
Edit: Sorry I was thinking in US gallons. Not UK. I apologize