reiverix said:I disagree. I use a python so it's just as easy to change any amount of water. I do 50% every week. The pH out my tap is around 7.4 and in my tank it is 6.6 with CO2. I refill the tank slowly, taking about 20 minutes. My fish love the water change.
Yeah well if you had a 180 gallon tank and it was 40F degrees outside that 50% water change in 20 minutes would drop the tempurature by 10-20 degrees and kill your fish. This is just one reason of many why 50% changes can stress and kill fish.
Otherwise if the water is the same then yeah a large water change wont hurt. For most of us though thats like a dream more than reality.