only problem is if you have chlorine in your water, even a little bit, 5 gallons a day can kill the benifical bacteria. you have to make sure there is none when doing water changes. With a large tank it takes longer before you need to change the water, so I just have been latly changing 10-20% every 2-3 weeks. or topping the tank off when water evaporates... This is just what I do on my 240...
MAD