OK, I'm checking out the fish profiles at (http://badmanstropicalfish.com/profile.html#profile) and for guppies it says to add 1tbsp of salt per gallon. I'm assuming this is still below the level of salt for brackish water??? How do you manage this with water changes then. Do you just guestimate how much water you are taking out and replacing? Is there a way to measure this little amount of salt after the fact to see if you've gotten off somehow?