Well first you will want the water to be a constant level. and a swc 120 is simply too small. Not enough water turnover. I would look at a external skimmer that will turnover enough water to match your volume.
The reason the water height in the sump varies is evaporation... I lose ~3g a day during the summer. What about a SRO-XP1000SS? That would fit in my sump... it would be a little underpowered for the system, but I don't feed more than once a day and my system is only lightly stocked.