My system is 400 gallons total, I have a 100g rubbermade that holds freshly mixed sea water, its heated to the same temp as the display and has its own blueLine 40 pump thats piped to keep the water circulating to mix the salt. When I do my changes I simply open the return valve to a drain and drain 80g of water from my system, I can tell exactly from the water level in my sump,As I drain the levels goes down to a point,I then shut the valve off on my return and close the re circ line in the make up water storage and open the feed line to the sump. With in 6 min the change is done. It cost me a little more to set it up this way but its well worth it. Your freashly mixed seawater is constanly being mixed, arivated & heated without thinking about it. The only thing I do is ad more salt to the mixing station when its drained. The system will re fill itself in 2 days with fresh RO water automaticly.