I am considering setting up a new reef, and was wondering what the evaporation would be from a Redsea XXL625, as I have noticed that there are no covers !
Evaporation will be different for everyone. The more surface agitation you have you will have more. If there are fans blowing on the surface you will have even more. My 220 will evaporate about 1/2 to 3/4 gallon a day without any fans...
Not exactly sure how you go about buying it though, you may need to order it through a Red Sea dealer or something. Also, this is a screen top, with a tank that large you want ample gas exchange on the surface of the water, so a glass top might be detrimental.
Evaporation will depend on how humid your air is and such, for example in the summer my evaporation on my 20 gallon is super slow, maybe 5 gallons in two months or so. In the winter when a cold front comes through and it gets super dry, it will do 2.5 gallons in a week.