<span class="link_user">@Bashgrowsitall1989</span>, thanks mate. Lights are about 3 feet away at the moment - due to be dropped to about 18 inches in the next day or 2.
I'm 96% sure the yellowing is from over watering. All the testing of my watering system has seen a lot more water go through than I would have liked - but yeah, it's all tinkering... and it's a lot of fun!
