Speaking of paranoid...the first thing that popped into my head after reading your post was the E. Coli spinach scare a year or so back, which was allegedly caused (or so some experts believe) by manure-rich run-off from cattle pastures contaminating nearby spinach fields. Of course, bacteria of all sorts, both naughty and nice, are normal residents of aquariums (including Mycobacterium marinum, among others). I've watered flower beds and rose bushes with old tank water, but I don't know that I'd water my herb garden with it. That's how paranoid I am! I'd be less worried about the Excel.