Biocube32 evaporation

So I haven't had a saltwater tank in about 12 years so I consider myself a noob. One thing I do remember from my old tanks is very basic add fresh rodi water to top of tank when your water evaporates .. on my old tanks( one was a 55 gallon with a canopy on top and a 15 gallon open sump on the bottom and one was a 30 gallon open top tank) water seemed to evaporate fairly quickly as in I added fresh top off water weekly. The strange thing about my new biocube 32 is I've had it up for about 4 weeks and haven't had to add any freshwater. I have been doing about 20 percent saltwater changes which I didn't do on my old tanks.. I use two different hydrometers and the sg is 1.023.. this is starting to concern me it seems like I should have had to add freshwater by now.. I do leave my thermostat in my home now at 70 degrees compared to 85 in the old days. Don't know if that's why I have less evaporation or maybe it's the lid on the biocube?? Any thoughts??

Thanks!!