You shouldn’t put your heater in the return chamber since that’s where any evaporation will show up. If you changed your return pump, the increased power will result in the water level in the DT to be slightly higher (as the weir is only so wide, the water level rises until it’s pushing through as much water as the return pump is pushing in). This will make the water level in the return chamber go down since it’s the chamber where any displacement or evaporation will show. This is completely normal in small AiO tanks as the overflow is sized to accommodate the volume from the stock return pump, when you get a more powerful pump, the overflow can’t accommodate the increased amount of water without raising the water level in the DT (think of it this way, if you push five 1’ square blocks through a doorway that was exactly 5’ wide, they fit just fine, but if you then tried to push 8 blocks through, you’d have to stack the extra 3 blocks on top of the 5 to fit them through the doorway - I feel like that doesn’t make any sense, hopefully you get what I’m saying). Just add some more salt water (you want the return chamber to be around half full, you don’t want it filled all the way to the top), mark the level that you fill it to (on my AiO, I have a little Velcro strip with a little arrow on a piece of Velcro, so I can move it up or down after a water change so I know how far to fill it up when I top the tank off).
