Another thing to try would be to put ice cubes in a ziplock bag and place them in the sump. Then you need not worry about SG
, and (or) introducing unprepared water into the tank. When they melt, then you just place it back into the freezer and when its ready put it back in. You can also have a few made up in advance.
125 Gal-35 Gal Wet/Dry-Tetra-Tec PF500 HOB
(3) AuquaClear 301 Powerheads-600 GPH
Automatic Tempature Controller
5-6 Inch Sand Bed-200+ lbs of LR