What redseareef said is true - you need to keep the sponge/filter cleaned every couple days to keep water level up in back. Plus if you have the gate too high that also drops the level in back.
Since you're talking about water level in the pump chamber, I assume you must mean the bottom inlet of the 2 RSM pumps? If that's true you can try adding a 3-4 inch piece of tubing to the inlets. But, the weight of the tubing will pull off that bottom section of the pump unless you secure them with a 12 inch plastic cable tie. Here's a post that discusses it:
Pump Mod 1
Here's a different type of pump mod by mbdave that sinks an MJ1200 pump down into the chamber.
Pump Mod 2
The first is probably easier. I've done that one, but it hasn't totally solved the water level problem for me. I think some of the air is pulled in through the cracks in the fittings. But, it has helped. The second mod is more involved but should get rid of all pump related microbubbles.
Someone once used aquarium safe silicone to seal all of the cracks in the pump fittings and that seemed to work, but the pumps would have to be dry, and the silicone given a couple days to cure. It also makes it difficult to take the pumps apart if they need cleaning.
