Cwesto, reading your problems post, I was expecting to see sick plants... THose are FAR from ill; they look EXCELLENT. The yellowng of an occasional lower leaf like the one shown is normal, especially if it is not getting a lot of light through a dense, healthy upper canopy.
But the fact that your runoff water was ass-nasty says you were ready for a preventative flush anyway. My grow-shop guy had me LICKING potting soil recently to check for metallic/salty taste... and sure enough, it was noticeable. Hahaha.
I hope you remembered the last step, which is to weakly feed immediately after the flush?
Your 'lookout' is adorable.