Sorry late getting back to you. I have no idea why your water level could change in your main display. As long as your drain is handling all the water your sending it, then the water level should be set by the overflow box, and any evaporation etc should only affect your sump level. Running one or more pumps at a steady rate shouldn’t affect the display as long as all of the water returned from the tank to the sump all goes through the same overflow.
The only issue would be if one of your drain lines gets clogged which should then direct the excess water through your overflow safety drain. Changes in the level (height) of water in your display would have to imply a change in the volume of water leaving or returning to the tank. So if your pumps are set to run at a constant GPH there shouldn’t be any change in the water level.