I change out 5 gallons every Saturday on my 55g. Maybe it's because I'm a newbie but I actually enjoy and look forward to doing it. Also I use Imagitarium Ocean Water, which I have scheduled for repeat delivery from Petco every couple weeks. Of course it's considerably more expensive than mixing my own or buying at my LFS, but extremely convenient and I believe it to be better than artificial mixes, so IMO it's worth the higher cost. If I had a large tank requiring a lot more water I probably couldn't go that route.