Capn, you aren't a dunce!!! These are good questions. I completely drain out the res., and if it's really mucky I wipe it down with H2O2 and water. I'll fill a bucket half up, take out the air stones and stick em in the bucket and scrub them with a brush to get the gunk off. I soak the water pump that connects to the chiller as well and scrub with the brush. If the res isn't too bad, I don't do it every time. Then I put in the new water + Flawless Finish and leave in for 48 hours, checking pH.
I don't know anything about your nutes; look em up on High Times or 420 mag., or see what opinions other growers have. I trust AN because they will talk to you on the phone about cannabis cultivation, they make their nutes specifically for cannabis, and they aren't part of some disgusting corporation like Monsanto (who owns Scotts, who owns General Hydroponics). Plus I am really getting into AN's pH perfect lines of nutrients, which I'm hoping will keep me much less dependent on pH meters and all the friggin expense that goes with them...