I had the same concerns about this as well. In the end I opted not to use the fill and drain kit. Now i just syphon off the water with a 50ft hose attached to a eheim crook then when its time to fill up again I just use the same hose attached to a universal tap connector.
Those things waste loads of water because you have the tap on while your draining!
I just use a hose with a pump in one end inside the tank and the other end on the taps with hot and cold misture, with an extra piece coming off to drain, I can post some photos if interested. Pump cost ten or fifteen quid on ebay!