Thanks for the response! I'm going to try and simplify the system by running a single 3/4" poly pipe to the water's edge and a matala manifold 3/4" input with two 1/2" outputs and will add a pressure gauge at the pump. The only place water could be leaking into the system will be where the pipes connect to the diffusers.

I did not add any check valves after reading that the membrane type diffuser acts as a check valve when not running.

This is a pretty old pond (probably 50 years) and is fed mainly by rainwater. I'm not sure if there is a spring at all. I am mainly trying to help stagnation issues and prevent fish kills. I know 2 diffusers may not be enough but I just want to try to get some water movement and then i can always make adjustments later.