dnjan said:
I assume that the water you would remove would be water that was surface-skimmed from the tank (via your overflow system), which many assume to contain a higher amount of suspended/disolved organics. If the water you add is a few degrees cooler than your tank water, this water would tend to sink to the bottom, and reduce the chance of it being removed right away by the surface skimming. This should increase your efficiency (in terms of nutrient export and trace mineral replacement) a bit for a continuous system like you describe.
I have found basically doing what you describe works well and I've added a few tricks. I don't agree with Randy's Article on small changes being effective. It will work but you toss out an awful lot of the fresh salt mix you put in with small changes and it is very inefficient. I agree that larger periodic changes done more in the manner you describe makes the change more effective.
I've semi automated my method by putting a small waste pump in my sump, a MaxiJet 900. I have read too many horror stories of tank melt downs from trying to over automate water changes or add make up water. My waste pump is controlled by a high level switch in my sump. When the level in the sump reaches a very high level the pump turns on and quickly pumps the excess water to the sewer drain.
To make my water change, I make up a batch of fresh salt water, normally about 40% of the system volume, and let it set over night in the makeup tank.
My fresh mix is a few degrees cooler and a slightly higher SG. My wet skimming drops my SG a little over a couple of weeks and I need to adjust it back up. I have a small transfer pump in the makeup tank that is plumbed to the top of my tank.
Making a water change is easy from this point. I turn off all the pumps to stop the strong tank mixing currents, with a master switch. The tank level drops to the tank wier over flow level.
I then turn on the fresh saltwater transfer pump and slowly add the new salt water into the top of the tank and I don't cause a lot of mixing action. It takes about 4 or 5 minutes to slowly transfer the fresh mix. The fresh salt mix sinks slowy to the bottom because of it's higher SG and lower temperature. The old water is slowly pushed to the top and over the overflow into the sump. As the sump fills the small waste pump automatically pumps the excess into the drain. When the fresh mix tank is empty I turn all the system circulation and filter pumps back on.
Even with the slight differential in SG and temperature of the fresh salt mix, SPS and softie coral polyups don't even close. I haven't lost any coral to shock. In fact, they all expand and bloom after a water change.
I find making water changes in this manner makes it very easy. It also assure me I am not just dumping back out alot of the fresh mix I am adding.
