Some people make a mark at desired water level. Others just know. Most importantly, you should constantly moniter your specific gravity (salinity) and adjust accordingly. I keep my tank full right to the level of the canopy or where the glass quits on my tank. When the level changes I am aware of it quickly because your lights shine through. Sort of my alarm mechanism so I don't forget to monitor it. Depending on your set-up, there can be lots of evaporation in just a day, so its important to keep watch.