I actually have never even checked mine either. As far as I'm concerned if it tastes good enough to drink out of the tap then it's probably good enought to make beer.
There was a device not to long ago that was a converted water filter that you could connect between your keg and tap and fill with whole hops and run your brew through it before it hits the glass. Never tried one but I've heard it works good for adding hop flavor and aroma to finished beers.
I reuse yeast all the time and don't have a problem with off-flavors. I remove the left-over yeast and put it in a gallon glass jar with a lid on it and let it sit in the fridge until I'm ready to use it, usually within 2 weeks or so. When I get ready to use it I take it out of the fridge and...
I have an attachment on my CFC that injects the O2 as it passes through to the fermenter, works great. Before I got it I used a SS stone and just gave it a shot in the fermenter.