There are many different types of toxins that build up in a tank. By doing a large weekly
WC you are ensuring that the water will have low levels of toxins and if your tap water is hard with a higher
kh you are also replacing buffers back into the tank water which is what keeps your ph stable. Large
WC's when done properly do not stress fish. In fact I actually add water back 1-3 degrees cooler than the tank water which often triggers many of my fish to spawn either that day or the next morning. My fish are always very active after a big
WC.