I use a Genesis Renew. It's by far the best auto water change device on the market. I have it change out 5 gallons a day but you can set it to do anywhere from 1-99 gallons a day, a week or in one continuous water change. It can change out up to 1 gallon every 2 to 3 minutes. Also, because of it's pair of 1 gallon metering bins, there is no other solution on the market with that level of maintenance free accuracy. It uses 1 metering bin for new salt water and another for waste water to insure that the same amount is added & removed every time.
You can see more on the Renew here:
http://genesisreefsystems.com/products/automatic-water-change-systems/