Thanks for the kind words. I just seem to have an understanding how it all flows.
To explain the evaporation situation:
Water evaporates everywhere in your system. The surface of the tank, the inside of the overflow, the surface of the sump... every minute of every day, some of it is changing and joining the atmosphere. So why doesn't everything lower equally?
Because of the way the water is flowing. If you have 100g of water going through the system, and it evaporates 5g, you only have 95g of water left. The return pump will keep pushing water up to the tank, so the tank stays full. Water overflows and goes down to the sump, so the skimmer section (and the refugium pictured above) stay full. The final compartment, called the return section, is what will be lacking those 5g of water.
Now if the tank evaporates a couple more gallons, it would be down to 93g of water total. At this point, the return pump is pretty much sitting in a 1" puddle of water. The pump is now sucking in air and no longer can pump water up to the tank. Which means the tank no longer can overflow any more water down to the sump, and the sump section will no longer have any extra water to pour over into the return section.
Let's say you left it like this for a few more days. Now the water in the main tank would drop in height, lower and lower than the overflow teeth. It would be obvious to your eyes.
The skimmer section would gradually lower as well as the refugium.
The pump would completely burn up and possible seize due to heat issues. Hopefully the circuit would trip before a fire started.
And if the circuit tripped, the tank would pretty much go dead, and now you've got a whole other issue: stagnant water. Do you really want me to continue with this line of thinking?
Now you should understand the flow and why evaporation only occurs visibly in the return section. Topping off daily avoids all these issues. And if you look around in this forum, there are threads about automatic top off devices that automate this feature so you don't have to dwell on it.