dont worry your head mate basically mawgan, all i do is place maybe 1 x 54 t5 unit at the rear. place a reflector underneath the light so that the light shines up the back. word of warning! place the light at the top of the substarte level or black out the substrate at the back of the tank. you don't want unneeded light hitting the substrate causing problems such as BGA etc. i've never experienced it, but could cause problems. i used a grad/blue back ground @ MA and i personally found it to be outstanding in the effect it gave. kind of like this but in real time..... one other thing. i've used the window diffuser stuff from wilkos, and this stuff is great on white backgrounds, but IME, not so great with black. It renderers the deep blacks, slightly grey ish. tell me about mate. I always left, thinking...." why the hell have my last 4 big tanks been away from home?"