How many buckets of water is 25% of a 10g aquarium?

I've done a lot of water changes due to the goldfish's mess. When the aquarium gets filled up with waste, and the water turns cloudy, do i still siphon out 25%? or the whole thing. Because, i tried it once and the water still turned dirty/cloudy. i used the gravel vacuum to suck up waste and wash the gravel. And i don't really know how much water is 25%