I have a 300 gallon 10 foot tank and before I redid everything I had a pair of 100g tanks plumped into the system in addition to the sump. One was for fresh water and one was the mixing tank. Normally the mixing tank was part of the system, but when I wanted to do a water change I would just flip two valves (one to bypass and one to drain). Once it was drained, I closed the drain and opened up the valve on the freshwater tank to fill the mixing tank. Add salt, mix for a day, then flip the valves back and you instantly have a 100g water change.
It ran ok for about a year. The biggest issue (besides the damn Rubbermaid tank cracking) was the debris that accumulated in the tank. Despite the ease of changing water, I eventually decided that it was too much work and I never really followed a consistent schedule. So I eventually ripped it down. I always felt that I never really needed the extra water volume and it reduced the skimmer efficiency.
What I have now is a 100g custom sump and a 14g remote DSB. Also I have 2 @ 55 gallon drums filled with RO/DI water and a 55 gallon drum filled with salt water.
Litermeter #1 adds:
A) 14 liters of water a day to the sump for evap (I have to adjust it once a week)
B) 1 liter of Kalc water a day to the sump (low alk/ca demand right now)
C) 1 liter of water a day to the QT for evap
Litermeter #2:
A) Adds 3 liters of salt water to the sump (I skim about a liter a day)
B) Moves 2 liters of salt water from the sump to the QT
C) Removes 2 liters of salt water from QT
It might sound a bit complicated, but my water changes are done 150 times a day creating a very stable environment. I never have to rely on myself to do anything other than mix new salter water about once a month. Since the two fresh water drums are above the salt water drum, making new salt involves opening a value for a minute or two, dumping in the salt, and turning on the mixing pump. If I ever install the float switches that I bought I will get emails whenever levels drop above or below thresholds.
-Randy