IF you have fish then an ammonia level of 1.0 is notgood. Do a 20% water change as soon as you have newly mixed saltwater ready (remember to allow 24 hours for the salt to properly mix).
Sure this will prolong your cycle to some degree but it could also safe the fish's life.
At the specific head height your 2100 is working at what is its flow in GPH
? For example if you pump has to push the water up 4' then whats RIO say the flow on the 2100 is at 4'? If its above 300GPH then you have a problem because the pump would be filling the tank faster than the overflow could drain. I dont use CPR overflows for the very reason your citing. Some have a problem with them maintaning syphon. Do you have the recommended powerhead on the overflow?
CPR recommends a powerhead to be attached to the overflow to 'maintain' syphon and to help keep the air out. Air is the worst thing you can have i nyour overflow because thats what will kill your syphon.
IF the flow is to slow thru the overflow air will accumulate so its better to run the overflow close to the 300GPH speed than at a speed alot slower.