SOMEthinsFISHY, With your tank size I would say less than a gallon a month is kind of low. If I had my skimmer set to take out dry foam like most people I would probably only be getting a few gallons per month, which is fine depending upon what you are trying to accomplish. My system is designed for the lowest nutrient levels possible. By skimming very wet I am taking out detritus and nutrients BEFORE they start breaking down in the tank and polluting the water. Even though my skimmate is very wet, it still has a brown tint to it, which means I am pulling out some nasty stuff. There is also a lot of detritus at the bottom of my collection bucket every time I empty it.
gatohoser and dajum, Determining evaporation rate and the amount being skimmed out is really quite simple. To determine how much is being skimmed out I just measure the amount of skimmate that accumulates in the collection bucket in 1 day. To measure the amount of evaporation I can just turn off the skimmer for 24 hours and turn off my top off system. Then I just measure how much water needs to be added to the tank to make up for the evaporation in that 24 hour period. I think you guys are forgetting about my top off system.
My LitermeterIII doses make-up water and kalk to the tank to make up for evaporation. My water exchange module connected to my LitermeterIII doses fresh saltwater to make up for the water that is being skimmed out.
Right now I have my system pretty much dialed in to the point where I'm not getting any salinity fluctuations. However, there will always be a need to correct for salinity over time. It is impossible to get the exact same evaporation rate every day due to factors such as humidity in the air or how often my sump fans are cycling on/off to control the water temp. It is also impossible to skim the exact same amount of water out everyday due to differences in water chemistry fluctuations and feeding. However, with 400 gallons of total system water volume, it takes a while before the salinity fluctuates. Right now I test my salinity weekly to make sure it stays stable. I may need to add a little extra salt or take out some tank water and replace with fresh water once a month to correct for any salinity fluctuations.
In the future I plan to purchase a conductivity probe for my Aquacontroller Pro. This way I will be able to "control" the salinity in the tank. If the salinity rises too high I can have it shut off the saltwater additions. If the salinity falls I can have it shut off the top off additions, etc.