It's on the sump since the water level of the tank stays constant with evaporation. Does the sump being in semi- darkness under the stand with doors closed matter? I would think not since it does fill and keep the level constant..
Algae doesn't grow on the sump walls due to the lack of light, Don't see bubbles either.
Every week when I do a water change, I need to unplug the ato since the water level drops in the sump until I replace the new water and I don't want the salinity to drop.