Just drip 40 Gallons per day, and don't do water changes anymore. If at the end of a week, you don't have low nitrates, then go to 60 gallons per day. Go to www.thefilterguys.biz, they sell everything you need to do it.
I agree with axs377. Water change is very dependent on how big your fish are and how much you feed. The most important reason for doing water change is rid nitrate. Although nitrate is no where near as toxic (to most fresh water) as ammonia and nitrite, it is still toxic in a high concentration for a prolong period of exposure. If you think that a 30% water change every 2 weeks is adequate, then check do a nitrate test of your tank water. If its above 40ppm, you need to do water change. If you really want to do less water change than you need to look into plant filtration such as hydroponics. I personally do 50% w/c every 4 days to all my tanks....even the ones with supplementary plant filtration. I just think that w/c are the best you can do for your fish
we get 40-50 ppm out the tap in the uk xracer lol, wish we had the option to keep it lower here!! i do 70-80% per week im pretty sure my nitrates dont go over 80ppm, renders rays and delicate community fish almost impossible to keep here without hma...
op as others said, keep you nitrates as close to your tap water (if thats what you are using) as you can... whats you tap water like out your way? anything over 100ppm can become deadly to weaker species.