Originally Posted by robert958
well, 30% of water change will be weekly and vary as it goes, mean that sometime 30% sometimes 40% and so on and fore. right? But just worry, where there is some instant that water change one fish will dead. I am sure that water change has stress this weak fish...
Did this actually happen to you, or is it just a general question?
What 1077 was getting at is that regular weekly water changes that are sufficient for the tank--and as I said originally, the number of fish, type of fish, any plants, size of tank all influence how often and how much water change is needed--if they are sufficient, the tank will be stable with respect to the water conditions. But not doing a water change one or two weeks, then doing a large one, can be detrimental. This is because the toxins in the water that would normally be removed every week are still there and build up; sometimes fish "manage" with this short-term and "appear" OK, but the sudden large change of water may cause the water parameters to shift drastically, harming the fish. There are several ways this can occur, I won't go into all that. Only say that regular (weekly) water changes that are sufficient in volume for the tank's biology will be healthier for the fish.
You can never change too much water. In nature fish live in water that is changing (for them) every second. Fish do not live in the same water in nature as they are forced to do in any aquarium. So the more water changes the better. However, most of us would have difficulty doing several every day, and provided the tank is balanced once a week works.