Its been 8 years since I started reefing so that question my seams silly. I have 125 mixed reef tank. Every 2 weeks I change around 20-25 gal. of water. For that I have 55 gal. tank. So after filing it up I have enough water for 2 water changes. I recently read a lot that it is not good to leave mixed saltwater stagnate. I mean without power head or some sort of water movement. When I do first change remaining 20-25 gal. stays in tank for at least another 2 weeks. I don't see any bad effects on my reef. What do you guys think? Is it ok or i should throw power had there? What is your experience? Thanks.