The water level in my sump keeps going down...

[scherzo]

Oh.. I noticed that your other tanks are FW tanks.

So you won't notice evaporation on those tanks as much because you are looking at the level of water for the ENTIRE tank.. so it seems like it is going slower.

On a sump setup.. the ONLY area that you see where the evaporation takes place is IN THE SUMP which is a tiny cross section of the tank.. so 3 inches of evaporation in your sump.. every few days is probably equal to the amount of water you are losing in your fresh water tanks.. but you are looking at a larger surface area.. so your freshwater display tank will only lose 1 centimeter...