Most ATO float switches used in sump-based systems are best located as close as possible to the return pump. This is generally the first place where the water level will decline as evaporation occurs. Other areas (such as in the display tank) would only see a level drop once the pressure on the return pump lessened enough to cause it to be unable to push as much water back into the tank. And at that point, you may risk running the motor dry. At the least, having the float in the DT would cause more of a salinity swing - which is antithesis to the purpose of an ATO, really.
Additionally, your display tank likely has (or should have) a good degree of surface agitation, which would cause problems with a float switch.