Oh, I see what you're asking now. The hydramist has the ability to run multiple maps. If you want the option of running just pump gas, you'll need to have 2 maps, one tuned for water/meth, and one tuned for just pump gas. You will have to pay for an additional tune, but if you do this you won't ever need to worry about always having the tank filled.
A word of caution, though; it is a pita when you run the system dry. Air gets into the system, and it takes a while for all of it to get out. If you spray meth before all the air escapes, your flow rate will decrease. This should trigger the failsafe, but if it doesn't you could damage your engine. Try not to let your tank run completely dry.