I have a rather strange method to doing my water changes... and its not as preffered by some people, but hey, opinions are like elbows, everyone's got 'em. I use my Python kit to fill water from one end of the tank while I siphon from the other end of the tank, so as to keep the water level near full. (I don't particularly like shutting down my filters every time I do a water change... its a pain in the butt to get one of 'em to fire back up) I know that I likely get a little fresh water in with the bad water while I'm siphoning, but I do a decent sized change to make up for it. As for the siphon that is taking water out of the tank, I have a 20 gallon rubbermaid storage bin (the dark blue series, roughneck I think they call them... its incredibly strong) that I drain about 15 gallons of water into at a time, at which point I shut off the sink, siphon out the rubbermaid with the Python (so I don't have to carry the d*** thing... its effing heavy with that much water in it!) and repeat the process until I've taken out a total of 20-25 gallons of water. I'll do this at least once a week, occasionally more often. (this is on a 55g tank)